Investments - Net Investment Income and Investment Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Total investment income	$ 1,878	$ 2,241	$ 2,194
Investment expenses	(61)	(59)	(59)
Investment gains (losses)	(57)	122	50
Net investment income	1,817	2,182	2,135
Fixed Maturity Securities [Member]
Net Investment Income [Line Items]
Total investment income	1,795	1,812	1,819
Investment gains (losses)	4	122	54
Limited Partnership Investments [Member]
Net Investment Income [Line Items]
Total investment income	22	277	199
Equity Securities [Member]
Net Investment Income [Line Items]
Total investment income	18	12	10
Investment gains (losses)	(74)		(5)
Short Term and Other [Member]
Net Investment Income [Line Items]
Total investment income	43	18	9
Investment gains (losses)	4	4	3
Derivative Instruments [Member]
Net Investment Income [Line Items]
Investment gains (losses)	9	(4)	(2)
Income (Loss) from Trading Portfolio [Member]
Net Investment Income [Line Items]
Total investment income	(54)	87	112
Other [Member]
Net Investment Income [Line Items]
Total investment income	$ 54	$ 35	$ 45

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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