Investments (Components of Net Investment Gains Losses) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Changes In Estimated Fair Value Subsequent To Purchase For Equity Securities	$ 31		$ (5)
Marketable Securities, Gain (Loss) [Abstract]
Fixed maturity securities — net gains (losses) on sales and disposals	(46)	$ 45	(141)	$ 43
Equity securities — net gains (losses) on sales and disposals	14	5	116	48
Change In Estimated Fair Value Of Equity Securities	20	0	(113)	0
Other net investment gains (losses):
Mortgage loans	(15)	(14)	(36)	(26)
Real estate and real estate joint ventures	89	270	114	267
Other limited partnership interests	8	(11)	8	(18)
Other	(58)	(70)	(188)	(96)
Subtotal	12	217	(240)	202
Change In Estimated Fair Value Of Other Limited Partnership Interests And Real Estate Joint Ventures	(1)	0	(6)	0
Non-investment portfolio gains (losses) (2)	(238)	(113)	(314)	(10)
Total net investment gains (losses)	(227)	104	(560)	192
State and political subdivision
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	(3)	0	(3)
Fixed Maturity Securities
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	(3)	0	(3)
Net investment gains (losses)	(46)	42	(141)	40
Equity Securities
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	(5)	0	(13)
Net investment gains (losses)	34	0	3	35
Nonredeemable Preferred Stock [Member]
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	0	0	(1)
Common Stock
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	$ 0	$ (5)	$ 0	$ (12)

Change In Estimated Fair Value Of Equity Securities.

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Change In Estimated Fair Value Of Other Limited Partnership Interests And Real Estate Joint Ventures.

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Changes In Estimated Fair Value Subsequent To Purchase For Equity Securities Still Held As Of The End Of The Period Included In Net Income Gains (Losses).

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The net gain (loss) resulting from sales and other disposals of other limited partnership interests.

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This item represents the subtotal of gains (losses)on the investment portfolio.

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Amount of realized and unrealized gain (loss) on investment.

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The difference between the carrying value and the sale price of debt securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale.

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The difference between the carrying value and the sale price of equity securities, not separately or otherwise categorized as trading or available-for-sale. This element includes investments in which the entity holds a small ownership stake (generally, less than 20% of the shares outstanding) and cannot exert significant influence.

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The net realized gain (loss) on investments sold during the period, not including gains (losses) on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, which, for cash flow reporting, is a component of proceeds from investing activities.

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Amount of gain (loss) on sale or disposal of other assets.

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The net gain (loss) resulting from a sale of loans, including adjustments to record loans classified as held-for-sale at the lower-of-cost-or-market and fair value adjustments to loan held for investment purposes.

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The net gain (loss) resulting from sales and other disposals of real estate owned for investment purposes.

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The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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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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