Stockholders' Equity - Change in AOCI Gain (Loss) by Component (Net of Tax) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	$ (616)	$ (430)	$ (872)
Other comprehensive income (loss) before reclassifications	(247)	(124)	477
Net realized (gains) losses reclassified from AOCI into earnings	(86)	(62)	(35)
Other comprehensive income (loss), net of tax	(333)	(186)	442
AOCI ending balance	(949)	(616)	(430)
Accumulated Net Unrealized Investment Gain (Loss) | Securities Available for Sale
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	162	410	106
Other comprehensive income (loss) before reclassifications	(172)	(268)	302
Net realized (gains) losses reclassified from AOCI into earnings	6	20	2
Other comprehensive income (loss), net of tax	(166)	(248)	304
AOCI ending balance	(4)	162	410
Accumulated Net Unrealized Investment Gain (Loss) | Securities Held to Maturity
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(725)	(821)	(897)
Other comprehensive income (loss) before reclassifications	0	0	0
Net realized (gains) losses reclassified from AOCI into earnings	104	96	76
Other comprehensive income (loss), net of tax	104	96	76
AOCI ending balance	(621)	(725)	(821)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	120	10	(110)
Other comprehensive income (loss) before reclassifications	(3)	284	228
Net realized (gains) losses reclassified from AOCI into earnings	(195)	(174)	(108)
Other comprehensive income (loss), net of tax	(198)	110	120
AOCI ending balance	(78)	120	10
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(143)	(8)	40
Other comprehensive income (loss) before reclassifications	(79)	(135)	(48)
Net realized (gains) losses reclassified from AOCI into earnings	0	0	0
Other comprehensive income (loss), net of tax	(79)	(135)	(48)
AOCI ending balance	(222)	(143)	(8)
Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(30)	(21)	(11)
Other comprehensive income (loss) before reclassifications	7	(5)	(5)
Net realized (gains) losses reclassified from AOCI into earnings	(1)	(4)	(5)
Other comprehensive income (loss), net of tax	6	(9)	(10)
AOCI ending balance	$ (24)	$ (30)	$ (21)