Stockholders' Equity - Change in AOCI Gain (Loss) by Component (Net of Tax) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	$ (934)	$ (41)	$ (949)	$ (616)
Other comprehensive income (loss) before reclassifications	256	307	290	903
Net realized (gains) losses reclassified from AOCI into earnings	(5)	(25)	(24)	(46)
Other comprehensive income (loss), net of tax	251	282	266	857
AOCI ending balance	(683)	241	(683)	241
Accumulated Net Unrealized Investment Gain (Loss) | Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	32	349	(4)	162
Other comprehensive income (loss) before reclassifications	147	137	183	319
Net realized (gains) losses reclassified from AOCI into earnings	2	(1)	2	4
Other comprehensive income (loss), net of tax	149	136	185	323
AOCI ending balance	181	485	181	485
Accumulated Net Unrealized Investment Gain (Loss) | Securities Held to Maturity
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(598)	(704)	(621)	(725)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Net realized (gains) losses reclassified from AOCI into earnings	23	25	46	46
Other comprehensive income (loss), net of tax	23	25	46	46
AOCI ending balance	(575)	(679)	(575)	(679)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(144)	497	(78)	120
Other comprehensive income (loss) before reclassifications	74	192	48	618
Net realized (gains) losses reclassified from AOCI into earnings	(29)	(49)	(69)	(98)
Other comprehensive income (loss), net of tax	45	143	(21)	520
AOCI ending balance	(99)	640	(99)	640
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(205)	(142)	(222)	(143)
Other comprehensive income (loss) before reclassifications	31	(30)	48	(29)
Net realized (gains) losses reclassified from AOCI into earnings	0	0	0	0
Other comprehensive income (loss), net of tax	31	(30)	48	(29)
AOCI ending balance	(174)	(172)	(174)	(172)
Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
AOCI beginning balance	(19)	(41)	(24)	(30)
Other comprehensive income (loss) before reclassifications	4	8	11	(5)
Net realized (gains) losses reclassified from AOCI into earnings	(1)	0	(3)	2
Other comprehensive income (loss), net of tax	3	8	8	(3)
AOCI ending balance	$ (16)	$ (33)	$ (16)	$ (33)