Investment Securities - Investment Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 26,701	$ 25,772
Unrealized Losses Recorded in AOCI	(989)	(1,153)
Carrying Value	25,712	24,619
Gross Unrealized Gains	683	779
Gross Unrealized Losses	(199)	(81)
Fair Value	26,196	25,317
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	199	199
Unrealized Losses Recorded in AOCI	0	0
Carrying Value	199	199
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Fair Value	199	198
Agency | RMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,022	22,561
Unrealized Losses Recorded in AOCI	(897)	(1,048)
Carrying Value	22,125	21,513
Gross Unrealized Gains	606	692
Gross Unrealized Losses	(158)	(72)
Fair Value	22,573	22,133
Agency | CMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,480	3,012
Unrealized Losses Recorded in AOCI	(92)	(105)
Carrying Value	3,388	2,907
Gross Unrealized Gains	77	87
Gross Unrealized Losses	(41)	(8)
Fair Value	$ 3,424	$ 2,986