| **Consolidated Balance Sheets - (Parenthetical) - USD ($) $ in Millions** | Dec. 31, 2017 | Dec. 31, 2016 |
|---------------------------------------------------------------------------|---------------|---------------|
| **Statement of Financial Position [Abstract]**                            |          |          |
| Fixed maturities securities at amortized cost                             | $ 38,215      | $ 38,361      |
| Equity securities at cost                                                 | 659           | 106           |
| Allowance for uncollectible reinsurance                                   | 29            | 37            |
| Allowance for uncollectible insurance receivables                         | 44            | 46            |
| Accumulated depreciation on property and equipment                        | 274           | 254           |
| Due to related parties                                                    | $ 143         | $ 50          |
| Common stock, par value (in usd per share)                                | $ 2.50        | $ 2.50        |
| Common stock, shares authorized (in shares)                               | 500,000,000   | 500,000,000   |
| Common stock, shares issued (in shares)                                   | 273,040,243   | 273,040,243   |
| Common stock, shares outstanding (in shares)                              | 271,205,390   | 270,495,998   |
| Treasury stock, shares (in shares)                                        | 1,834,853     | 2,544,245     |

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Marketable securities, fixed maturities, at amortized cost
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Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.
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This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
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Face amount or stated value per share of common stock.
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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.
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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.
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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.
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Carrying amount as of the balance sheet date of obligations due all related parties.
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The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value.
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Amount of valuation allowance for reinsurance recoverables. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.
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Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends.
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