Schedule I - Parent Company Financial Information (Condensed Statements of Comprehensive Income) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income	$ 184	$ 499	$ 650	$ 1,115	$ (401)	$ 450	$ 311	$ 254	$ 2,448	$ 614	$ 1,276
Other comprehensive income (loss):
Change in gross unrealized investment gains/losses									149	(101)	(114)
Effect of income taxes									(55)	38	42
Total change in unrealized investment gains/losses, net of tax									94	(63)	(72)
Reclassification adjustment for net realized gains included in investment income									(14)	(96)	(146)
Effect of income taxes									5	35	53
Total reclassification adjustment, net of tax									(9)	(61)	(93)
Other comprehensive income (loss), net of tax									85	(124)	(165)
Comprehensive income									2,533	490	1,111
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									2,448	614	1,276
Other comprehensive income (loss):
Change in gross unrealized investment gains/losses									149	(101)	(114)
Effect of income taxes									(55)	38	42
Total change in unrealized investment gains/losses, net of tax									94	(63)	(72)
Reclassification adjustment for net realized gains included in investment income									(14)	(96)	(146)
Effect of income taxes									5	35	53
Total reclassification adjustment, net of tax									(9)	(61)	(93)
Other comprehensive income (loss), net of tax									85	(124)	(165)
Comprehensive income									$ 2,533	$ 490	$ 1,111