Investments - Net Investment Income and Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Investment Income [Line Items]
Total investment income	$ 567	$ 493	$ 1,088	$ 1,111
Investment expenses	(16)	(15)	(31)	(29)
Investment gains (losses)	(3)	43	6	77
Net investment income	551	478	1,057	1,082
Fixed Maturity Securities [Member]
Net Investment Income [Line Items]
Total investment income	444	457	890	912
Investment gains (losses)	4	44	22	76
Equity Securities [Member]
Net Investment Income [Line Items]
Total investment income	12	2	22	3
Investment gains (losses)	(10)		(25)
Derivative Instruments [Member]
Net Investment Income [Line Items]
Investment gains (losses)	4	(3)	9	(2)
Short Term and Other [Member]
Net Investment Income [Line Items]
Total investment income	11	4	20	8
Investment gains (losses)	(1)	2		3
Limited Partnership Investments [Member]
Net Investment Income [Line Items]
Total investment income	60	23	108	139
Income (Loss) from Trading Portfolio [Member]
Net Investment Income [Line Items]
Total investment income	23	(1)	20	33
Other [Member]
Net Investment Income [Line Items]
Total investment income	$ 17	$ 8	$ 28	$ 16

Net Investment Income And Gains Losses On Trading Securities Before Investment Expenses

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

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Amount, before investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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