|**General (Policies)**                              | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      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|**Accounting Policies [Abstract]**                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Consolidation                                       | The accompanying Condensed Consolidated Financial Statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP).  Intercompany amounts have been eliminated.  Certain financial information that is normally included in annual financial statements prepared in accordance with GAAP, including certain financial statement notes, is not required for interim reporting purposes and has been condensed or omitted.  These statements should be read in conjunction with the Consolidated Financial Statements and notes thereto included in CNAF's Annual Report on Form 10-K filed with the Securities and Exchange Commission for the year endedDecember 31, 2017, including the summary of significant accounting policies in NoteA.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Use of Estimates                                    | The preparation of Condensed Consolidated Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the Condensed Consolidated Financial Statements and the reported amounts of revenues and expenses during the reporting period.  Actual results may differ from those estimates.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Recently Adopted Accounting Standards Updates (ASU) | Recently Adopted Accounting Standards Updates (ASU)ASU 2014-09:In May 2014, the Financial Accounting Standards Board (FASB) issued ASU No. 2014-09,Revenue Recognition (Topic 606):Revenue from Contracts with Customers.  The standard excludes from its scope the accounting for insurance contracts, financial instruments, and certain other agreements that are governed under other GAAP guidance.  The updated guidance requires an entity to recognize revenue as performance obligations are met, in an amount that reflects the consideration the entity is entitled to receive for the transfer of the promised goods or services.On January 1, 2018, the Company adopted the updated guidance using the modified retrospective method applied to all contracts which were not completed as of the date of adoption, with the cumulative effect recognized as an adjustment to the opening balance of Retained earnings.  Prior period amounts have not been adjusted and continue to be reported in accordance with the previous accounting guidance.Under the new guidance, revenue on warranty products and services will be recognized more slowly compared to the historic revenue recognition pattern.  In addition, for warranty products in which the Company acts as the principal in the transaction, Non-insurance warranty revenue and Non-insurance warranty expense are increased to reflect the gross amount paid by consumers, including the retail seller’s markup which is considered a commission to the Company's agent.  This gross-up of revenue and expense also resulted in an increase to Other assets and Other liabilities on the Company's Condensed Consolidated Balance Sheets as the revenue and expense are recognized over the actuarially determined expected claims emergence pattern.The cumulative effect changes to the Condensed Consolidated Balance Sheet for the adoption of the updated guidance onJanuary 1, 2018were as follows:(In millions)Balance as of December 31, 2017Adjustments Due to Adoption of Topic 606Balance as of January 1, 2018Other assets$1,133$1,882$3,015Other liabilities4,2531,9696,222Deferred income taxes13721158Retained earnings9,414(66)9,348The impact of adoption on the Condensed Consolidated Statement of Operations and Balance Sheet as of and for the three months endedMarch 31, 2018was as follows:Three months ended March 312018 Prior to AdoptionEffect of Adoption2018 as Reported(In millions)Statement of operations:Non-insurance warranty revenue$101$137$238Total revenues2,3981372,535Non-insurance warranty expense78138216Total claims, benefits and expenses2,0511382,189Income before income tax347(1)346Income tax expense(55)—(55)Net income292(1)291Balance sheet(1):Other assets$3,154$87$3,241Other liabilities6,321886,409Deferred income taxes238—238Retained earnings9,029(1)9,028(1)2018 Prior to Adoption includes the cumulative effect adjustment at adoption.See NoteJto the Condensed Consolidated Financial Statements for additional information regarding non-insurance revenues from contracts with customers.ASU 2016-01:In January 2016, the FASB issued ASU No. 2016-01,Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities.  The updated accounting guidance requires changes to the reporting model for financial instruments.  The guidance primarily changes the model for equity securities by requiring changes in the fair value of equity securities (except those accounted for under the equity method of accounting, those without readily determinable fair values and those that result in consolidation of the investee) to be recognized through the income statement.  The Company adopted the updated guidance onJanuary 1, 2018and recognized a cumulative effect adjustment that increased beginning Retained earnings by$28 million, net of tax.  Prior period amounts have not been adjusted and continue to be reported in accordance with the previous accounting guidance.For the three months endedMarch 31, 2018, the Company recognized a$15 millionpretax loss within Net realized investment gains (losses) for the change in fair value of non-redeemable preferred stock and less than a$1 millionpretax gain within Net investment income for the change in fair value of common stock as a result of this change.  For the three months endedMarch 31, 2017a$1 milliondecrease in the fair value of common stock and a$5 millionincrease in the fair value of non-redeemable preferred stock was recognized in Other comprehensive income.  The Company's non-redeemable preferred stock contain characteristics of debt securities, are priced similarly to bonds, and are held primarily for income generation through periodic dividends.  While recognition of gains and losses on these securities are no longer discretionary, management does not consider the changes in fair value of non-redeemable preferred stock to be reflective of our primary operations.  As such, the changes in the fair value of these securities are recorded through Net realized investments gains (losses).  During the first quarter of 2018, the Company increased its common stock with the intention of holding the securities primarily for market appreciation.  As such, the changes in the fair value of these securities are recorded through Net investment income.ASU 2017-07:In March 2017, the FASB issued ASU No. 2017-07,Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. The updated accounting guidance requires changes to the presentation of the components of net periodic benefit cost on the income statement by requiring service cost to be presented with other employee compensation costs and other components of net periodic pension cost to be presented outside of any subtotal of operating income. The ASU also stipulates that only the service cost component of net benefit cost is eligible for capitalization.  The Company adopted the updated guidance effective January 1, 2018.  The guidance was applied on a prospective basis for capitalization of service costs and on a retrospective basis for the presentation of the service cost and other components of net periodic benefit costs in the Company's Condensed Consolidated Statements of Operations and in its disclosures.  The Company expanded the related footnote disclosure, NoteGto the Condensed Consolidated Financial Statements, to disclose the amount of service cost and non-service cost components of net periodic benefit cost and the line items in the Condensed Consolidated Statements of Operations in which such amounts are reported.  The change limiting the costs eligible for capitalization is not material to the Company’s results of operations or financial position.ASU 2018-02:In February 2018, the FASB issued ASU No. 2018-02,Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income.GAAP requires the remeasurement of deferred tax assets and liabilities due to a change in the tax rate to be included in Net income, even if the related income tax effects were originally recognized in Accumulated other comprehensive income (AOCI). The ASU allows a reclassification from AOCI to Retained earnings for stranded tax effects resulting from the new U.S. Federal corporate income tax rate enacted on December 22, 2017.  The Company early adopted the updated guidance effective January 1, 2018 and elected to reclassify the stranded income tax effects relating to the reduction in the Federal corporate income tax rate from AOCI to Retained earnings at the beginning of the period of adoption. The net impact of the accounting change resulted in a$12 millionincrease in AOCI and a corresponding decrease in Retained earnings. The$12 millionincrease in AOCI is comprised of a$142 millionincrease in net unrealized gains (losses) on investments partially offset by a$130 milliondecrease in unrecognized pension and postretirement benefits.The Company releases tax effects from AOCI utilizing the security-by-security approach for Net unrealized gains (losses) on investments with Other-than-temporary impairment (OTTI) losses and Net unrealized gains (losses) on other investments. For Pension and postretirement benefits, tax effects from AOCI are released at enacted tax rates based on the pre-tax adjustments to pension liabilities or assets recognized within OCI.Accounting Standards Pending AdoptionIn February 2016, the FASB issued ASU No. 2016-02,Leases (Topic 842): Accounting for Leases.The updated accounting guidance requires lessees to recognize on the balance sheet the assets and liabilities for the rights and obligations created by all leases, including those historically accounted for as operating leases.  The guidance is effective for interim and annual periods beginning after December 15, 2018.  The Company is currently evaluating the effect the updated guidance will have on the Company's financial statements.  It is expected that assets and liabilities will increase based on the present value of remaining lease payments for leases in place at the adoption date; however, this is not expected to be material to the Company's results of operations or financial position.In June 2016, the FASB issued ASU No. 2016-13,Financial Instruments-Credit Losses (Topic 326): Measurementof Credit Losses on Financial Instruments.The updated accounting guidance requires changes to the recognition of credit losses on financial instruments not accounted for at fair value through net income.  The guidance is effective for interim and annual periods beginning after December 15, 2019.  The Company is currently evaluating the effect the guidance will have on the Company's financial statements, but expects the primary changes to be the use of the expected credit loss model for its mortgage loan portfolio and reinsurance receivables and the presentation of credit losses within the available-for-sale fixed maturities portfolio through an allowance method rather than as a direct write-down.  The expected credit loss model will require a financial asset to be presented at the net amount expected to be collected.  The allowance method for available-for-sale debt securities will allow the Company to record reversals of credit losses if the estimate of credit losses declines.Income Tax Reform UpdateOn December 22, 2017, H.R.1, “An Act to Provide for Reconciliation Pursuant to Titles II and V of the Concurrent Resolution on the Budget for Fiscal Year 2018,” was signed into law (Tax Reform Legislation).Shortly after enactment, the Securities and Exchange Commission Staff issued Staff Accounting Bulletin No. 118 (SAB 118) to provide guidance on accounting for the Tax Reform Legislation impacts when the measurements of the income tax effects are complete, incomplete, or incomplete but for which a provisional amount can be estimated.  SAB 118 permits the recognition of provisional amounts, and adjustments to provisional amounts, in subsequent reporting periods within the one year measurement period.The Company has reflected the following incomplete but reasonably estimated provisional items in Deferred income taxes on the Condensed Consolidated Balance Sheet at March 31, 2018.  The effects of the adjustments to the Company’s provisional amounts for the three months ended March 31, 2018 did not impact income tax expense.•The Company has recalculated its insurance reserves and the transition adjustment from existing law.•The Company has recalculated amounts under special accounting method provisions for recognizing income for Federal income tax purposes no later than for financial accounting purposes and the transition adjustment from existing law.•The Company has not recorded current or deferred taxes with respect to the international provisions since it does not expect to have inclusions in U.S. taxable income for certain earnings of foreign subsidiaries in future years.
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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|(In millions)                                       | Balance as of December 31, 2017                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   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|Deferred income taxes                               | 137                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         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|Deferred income taxes                               | 238                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Retained earnings                                   | 9,029                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|(1)                                                 | 2018 Prior to Adoption includes the cumulative effect adjustment at adoption.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              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                                                 
|•                                                   | The Company has recalculated its insurance reserves and the transition adjustment from existing law.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       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                                                 
|•                                                   | The Company has recalculated amounts under special accounting method provisions for recognizing income for Federal income tax purposes no later than for financial accounting purposes and the transition adjustment from existing law.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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                                                 
|•                                                   | The Company has not recorded current or deferred taxes with respect to the international provisions since it does not expect to have inclusions in U.S. taxable income for certain earnings of foreign subsidiaries in future years.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Earnings (Loss) Per Share                           | Earnings (loss) per share is based on the weighted average number of outstanding common shares.  Basic earnings (loss) per share excludes the impact of dilutive securities and is computed by dividing Net income (loss) by the weighted average number of common shares outstanding for the period.  Diluted earnings (loss) per share reflects the potential dilution that could occur if securities or other contracts to issue common stock were exercised or converted into common stock.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 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