| **Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Thousands, $ in Millions**         | 3 Months Ended | 9 Months Ended | 12 Months Ended   |                   |
|---------------------------------------------------------------------------------------------------------------------------|----------------|----------------|-------------------|-------------------|
||
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Retained earnings                                                                                                         |           |           | $ 9,663           | $ 9,081           |   |   |
| Mortgage loans on real estate commercial and consumer allowance for credit loss                                           |           |           | 16                | 26                |   |   |
| Deferred income taxes                                                                                                     |           |           | 142               | 66                |   |   |
| Marketable securities fixed maturities allowance for credit loss                                                          |           |           | 18                | 40                |   |   |
| Anticipated amounts due from insureds related to losses under deductible policies                                         |           |           | $ 1,100           | $ 1,200           |   |   |
| Structured settlement annuities, interest rate, low end                                                                   |           |           | 6.40%             |              |   |   |
| Structured settlement annuities, interest rate, high end                                                                  |           |           |              | 6.50%             |   |   |
| Discounted reserves for unfunded structured settlements                                                                   |           |           | $ 503             | $ 520             |   |   |
| Discounted reserves for unfunded structure settlements, discount amount                                                   |           |           | 621               | 657               |   |   |
| Amount of interest recognized on the discounted reserves of unfunded structured settlements                               |           |           | $ 36              | $ 35              | $ 36 |   |
| Interest rates at which workers' compensation lifetime claim reserves are discounted at                                   |           |           | 3.50%             | 3.50%             |   |   |
| Workers' compensation liability                                                                                           |           |           | $ 228             | $ 258             |   |   |
| Discounted reserves for workers’ compensation lifetime claim reserves, discount amount                              |           |           | 97                | 113               |   |   |
| Amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves |           |           | $ 12              | $ 15              | $ 21 |   |
| Long term care claim reserves weighted average discounted interest rate                                                   |           |           | 5.80%             | 5.80%             |   |   |
| Discounted reserves for long term care claim reserves                                                                     |           |           | $ 2,700           | $ 2,700           |   |   |
| Premium deficiency                                                                                                        |           |           | $ 428             | $ 439             |   |   |
| Liability for future policy benefits, gross premium valuation, pretax margin                                              | $ 72           | $ 72           |              |              |   |   |
| Minimum interest rate used to calculate reserves for long term care products                                              |           |           | 5.30%             | 5.40%             |   |   |
| Liability balance for guaranty fund                                                                                       |           |           | $ 79              | $ 82              |   |   |
| Percentage of billed receivables compared to total reinsurance receivable                                                 |           |           | 5.00%             |              |   |   |
| Policyholder dividends, rate on policy earnings                                                                           |           |           | 1.00%             | 1.00%             | 1.00% |   |
| Increase (decrease) in shadow adjustment, net of tax                                                                      |           |           | $ (296)           | $ 575             |   |   |
| Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments                               |           |           | 2,477             | 2,773             |   |   |
| Foreign currency transaction gain (loss), before tax                                                                      |           |           | $ (1)             | $ 13              | $ 1 |   |
| Operating lease, right-of-use asset, statement of financial position [Extensible List]                                    |           |           | Other assets      | Other assets      |   |   |
| Operating lease, liability, statement of financial position [Extensible List]                                             |           |           | Other liabilities | Other liabilities |   |   |
| Weighted average number diluted shares outstanding adjustment (in shares)                                                 |           |           | 1,000             | 770               | 960 |   |
| Antidilutive securities excluded from computation of earnings per share, amount                                           |           |           | 1                 | 8                 | 1 |   |
| Interest paid, net                                                                                                        |           |           | $ 110             | $ 124             | $ 136 |   |
| Income taxes paid                                                                                                         |           |           | $ 278             | $ 108             | $ 255 |   |
| Furniture and Fixtures                                                                                                    |           |           |              |              |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Property, plant and equipment, useful life                                                                                |           |           | 7 years           |              |   |   |
| Office Equipment                                                                                                          |           |           |              |              |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Property, plant and equipment, useful life                                                                                |           |           | 5 years           |              |   |   |
| Computer Software, Intangible Asset                                                                                       |           |           |              |              |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Property, plant and equipment, useful life                                                                                |           |           | 10 years          |              |   |   |
| Minimum                                                                                                                   |           |           |              |              |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Warranty coverage term                                                                                                    |           |           | 1 month           |              |   |   |
| Requisite service period for stock- based compensation expense                                                            |           |           | 3 years           |              |   |   |
| Minimum | Computer Software, Intangible Asset                                                                             |           |           |              |              |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Property, plant and equipment, useful life                                                                                |           |           | 3 years           |              |   |   |
| Maximum                                                                                                                   |           |           |              |              |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Warranty coverage term                                                                                                    |           |           | 10 years          |              |   |   |
| Maximum | Computer Software, Intangible Asset                                                                             |           |           |              |              |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Property, plant and equipment, useful life                                                                                |           |           | 5 years           |              |   |   |
| ASU 2016-13                                                                                                               |           |           |              |              |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Retained earnings                                                                                                         |           |           |              |              |   | $ (5) |
| Mortgage loans on real estate commercial and consumer allowance for credit loss                                           |           |           |              |              |   | 7 |
| Deferred income taxes                                                                                                     |           |           |              |              |   | 2 |
| Marketable securities fixed maturities allowance for credit loss                                                          |           |           |              |              |   | $ 6 |
| CNAF Consolidated | Loews                                                                                                 |           |           |              |              |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                          |           |           |              |              |   |   |
| Noncontrolling interest, ownership percentage by parent                                                                   |           |           | 89.60%            |              |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves
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Amount of interest recognized on the discounted reserves of unfunded structured settlements
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| X                                                                                                                                                                                                                                                                                           |
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| - Definition
Losses under deductible policies
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Discounted reserves for unfunded structure settlements, discount amount
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Discounted reserves for unfunded structured settlements
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| X                                                                                                                                                                                                                                                                                        |
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| - Definition
Discounted reserves for long term care claim reserves
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Discounted reserves for long term care claim reserves, discount amount
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Increase (decrease) in shadow adjustment, net of tax
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| - Definition
Liability for Future Policy Benefits, Gross Premium Valuation, Pretax Margin
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No definition available.
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| - Definition
Long term Care Claim Reserves Weighted Average Discounted Interest Rate
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| - Definition
Marketable Securities Fixed Maturities Allowance For Credit Loss
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| - Definition
Mortgage Loans on Real Estate Commercial and Consumer Allowance For Credit Loss
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| - Definition
Percentage Of Billed Receivables Compared To Total Reinsurance Receivable
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum interest rate in the range of interest rates used to calculate future benefits to be paid to or on behalf of policyholders and related expenses less the present value of future net premiums using assumptions such as estimates of expected investment yields, mortality, morbidity, terminations and expenses, applicable at the time the insurance contracts are made and cost of settling insured claims and cost incurred in the claim settlement process of structured settlement annuities.
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| - Definition
Structured Settlement Annuities, Interest Rates, Low End
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| - Definition
Warranty Coverage Term
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.
+ References
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| - Definition
Amount, after allocation of valuation allowances and deferred tax liability, of deferred tax asset attributable to deductible differences and carryforwards, with jurisdictional netting.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement.
+ References
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| - Definition
The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash paid for interest, excluding capitalized interest, classified as operating activity. Includes, but is not limited to, payment to settle zero-coupon bond for accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The assumed interest rate used to determine present value of future benefits to be paid to or on behalf of policyholders and related expenses less the present value of future net premiums using assumptions such as estimates of expected investment yields, mortality, morbidity, terminations and expenses, applicable at the time the insurance contracts are made and unpaid claim costs relating to insurance contracts including estimates of costs relating to incurred but not reported claims based on the estimated ultimate cost of settling the claims (including effects of inflation and other societal and economic factors), using past experience adjusted for current trends and any other factors that would modify past experience in the year the policy went into force.
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No definition available.
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| - Definition
The liability as of the balance sheet date representing required funding mandated by statute or regulatory authority that is related directly or indirectly to underwriting activities, including pools for self-insurance and excluding premium taxes and income taxes. The funds derived from the assessments generally enable a government guarantor to pay insurance claims in the event that the underwriting insurer is insolvent or otherwise unable to meet its payment obligations.
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| - Definition
The parent entity's interest in net assets of the subsidiary, expressed as a percentage.
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No definition available.
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
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No definition available.
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| - Definition
Indicates line item in statement of financial position that includes operating lease liability.
+ References
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| - Definition
Indicates line item in statement of financial position that includes operating lease right-of-use asset.
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| - Definition
The percentage of earnings allocable to participating policyholders based on earnings of such policies for payment of dividends.
+ References
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| - Definition
Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.
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No definition available.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cumulative amount of the reporting entity's undistributed earnings or deficit.
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| - Definition
Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
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| - Definition
The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.
+ References
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| - Definition
Current year monetary amount of the change in worker's compensation reserve liability resulting from the discount applied to reduce the reserve to present value.
+ References
No definition available.
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| - Definition
Percent discount applied to worker's compensation reserve liability to reduce the reserve to present value.
+ References
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| - Definition
Carrying value as of the balance sheet date of obligations and payables pertaining to claims incurred of a workers compensation nature.
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