Schedule I - Parent Company Financial Information (Condensed Statements of Comprehensive Income) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net income	$ 101	$ 314	$ 431	$ 430	$ 145	$ 290	$ 344	$ 368	$ 1,276	$ 1,147	$ 1,231
Other comprehensive (loss) income:
Change in gross unrealized investment gains/losses									(114)	122	(338)
Effect of income taxes									42	(44)	124
Total change in unrealized investment gains/losses, net of tax									(72)	78	(214)
Reclassification adjustment for net realized gains included in investment income									(146)	(20)	(22)
Effect of income taxes									53	7	8
Total reclassification adjustment, net of tax									(93)	(13)	(14)
Other comprehensive (loss) income, net of tax									(165)	65	(228)
Comprehensive income									1,111	1,212	1,003
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									$ 1,276	$ 1,147	$ 1,231