Investments - Net Investment Income and Investment Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Investment Income [Line Items]
Total investment income	$ 569	$ 567	$ 1,245	$ 1,088
Investment expenses	(18)	(16)	(37)	(31)
Investment gains (losses)	2	(3)	33	6
Net investment income	551	551	1,208	1,057
Fixed Maturity Securities [Member]
Net Investment Income [Line Items]
Total investment income	455	444	910	890
Investment gains (losses)	(3)	4	(9)	22
Limited Partnership Investments [Member]
Net Investment Income [Line Items]
Total investment income	43	60	124	108
Short Term and Other [Member]
Net Investment Income [Line Items]
Total investment income	14	11	29	20
Investment gains (losses)		(1)
Equity Securities [Member]
Net Investment Income [Line Items]
Total investment income	16	12	46	22
Investment gains (losses)	11	(10)	53	(25)
Income From Trading Portfolio [Member]
Net Investment Income [Line Items]
Total investment income	29	23	110	20
Other [Member]
Net Investment Income [Line Items]
Total investment income	12	17	26	28
Derivative Instruments [Member]
Net Investment Income [Line Items]
Investment gains (losses)	$ (6)	$ 4	$ (11)	$ 9

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

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