| **Debt**                       | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
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| **Debt Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| Debt Disclosure [Text Block]   | Note I. DebtDebt is composed of the following obligations.December 31   (In millions)2014 2013Short term debt:   Senior notes of CNAF, 5.850%, face amount of $549, due December 15, 2014$— $549    Long term debt:   Senior notes of CNAF:   6.500%, face amount of $350, due August 15, 2016349 3496.950%, face amount of $150, due January 15, 2018150 1497.350%, face amount of $350, due November 15, 2019348 3485.875%, face amount of $500, due August 15, 2020497 4975.750%, face amount of $400, due August 15, 2021397 3973.950%, face amount of $550, due May 15, 2024547 —Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023241 241Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 203630 30Total long term debt2,559 2,011Total debt$2,559 $2,560CCC is a member of the Federal Home Loan Bank of Chicago (FHLBC).  FHLBC membership provides participants with access to additional sources of liquidity through various programs and services.  As a requirement of membership in the FHLBC, CCC held $16 million of FHLBC stock as of December 31, 2014 and 2013 giving it access to approximately $330 million of additional liquidity.  As of December 31, 2014 and 2013, CCC had no outstanding borrowings from the FHLBC. The Company has a four-year $250 million senior unsecured revolving credit facility with a syndicate of banks which is intended to be used for general corporate purposes.  At the Company's election, the commitments under the credit agreement may be increased from time to time up to an additional aggregate amount of $100 million.  Under the credit agreement, the Company is required to pay a facility fee which would adjust automatically in the event of a change in the Company's financial ratings.  The credit agreement includes several covenants, including maintenance of a minimum consolidated net worth and a specified ratio of consolidated indebtedness to consolidated total capitalization.  The minimum consolidated net worth, as defined as of December 31, 2014, was $8.6 billion.  As of December 31, 2014 and 2013, we had no outstanding borrowings under the credit agreement.  The credit facility expires in 2016.The Company's debt obligations contain customary covenants for investment grade issuers.  The Company was in compliance with all covenants as of and for the years ended December 31, 2014 and 2013.The combined aggregate maturities for debt at December 31, 2014 are presented in the following table.Maturity of Debt(In millions) 2015$—20163502017—20181502019350Thereafter1,723Less discount(14)Total$2,559 |

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| - Definition
The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20,22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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