Stockholders' Equity - Change in AOCI Gain (Loss) by Component (Net of Tax) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	$ (949)	$ (616)	$ (430)
Other comprehensive income (loss) before reclassifications	81	(247)	(124)
Net realized (gains) losses reclassified from AOCI into earnings	(58)	(86)	(62)
Other comprehensive income (loss), net of tax	23	(333)	(186)
AOCI ending balance	(926)	(949)	(616)
Accumulated Net Unrealized Investment Gain (Loss) | Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(4)	162	410
Other comprehensive income (loss) before reclassifications	62	(172)	(268)
Net realized (gains) losses reclassified from AOCI into earnings	(41)	6	20
Other comprehensive income (loss), net of tax	21	(166)	(248)
AOCI ending balance	17	(4)	162
Accumulated Net Unrealized Investment Gain (Loss) | Securities Held to Maturity
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(621)	(725)	(821)
Other comprehensive income (loss) before reclassifications	0	0	0
Net realized (gains) losses reclassified from AOCI into earnings	97	104	96
Other comprehensive income (loss), net of tax	97	104	96
AOCI ending balance	(524)	(621)	(725)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(78)	120	10
Other comprehensive income (loss) before reclassifications	(95)	(3)	284
Net realized (gains) losses reclassified from AOCI into earnings	(108)	(195)	(174)
Other comprehensive income (loss), net of tax	(203)	(198)	110
AOCI ending balance	(281)	(78)	120
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(222)	(143)	(8)
Other comprehensive income (loss) before reclassifications	84	(79)	(135)
Net realized (gains) losses reclassified from AOCI into earnings	0	0	0
Other comprehensive income (loss), net of tax	84	(79)	(135)
AOCI ending balance	(138)	(222)	(143)
Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(24)	(30)	(21)
Other comprehensive income (loss) before reclassifications	30	7	(5)
Net realized (gains) losses reclassified from AOCI into earnings	(6)	(1)	(4)
Other comprehensive income (loss), net of tax	24	6	(9)
AOCI ending balance	$ 0	$ (24)	$ (30)