Investments - Realized Investment Gains and Losses and Change in Unrealized Investment Gains and Losses and Other Items (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Investment income summary:
Interest	$ 109	$ 105
Dividends	37	36
Other	1	0
Total	147	141
Less investment expenses	2	2
Total	145	139
Realized investment gains and losses summary:
Realized investment gains, net	61	47
Change in unrealized gains and losses summary:
Fixed maturities	190	(28)
Less income taxes	(100)	15
Fixed Maturities
Realized investment gains and losses summary:
Gross realized gains	3	3
Gross realized losses	(1)	0
Other-than-temporary impairments	(2)	0
Change in unrealized gains and losses summary:
Fixed maturities	116	46
Equity Securities
Realized investment gains and losses summary:
Gross realized gains	62	44
Gross realized losses	(1)	(1)
Other-than-temporary impairments	0	0
Other	0	1
Change in unrealized gains and losses summary:
Fixed maturities	$ 174	$ (89)