Investment Securities - Investment Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 25,772	$ 23,804
Unrealized Losses Recorded in AOCI	(1,153)	(1,304)
Carrying Value	24,619	22,500
Gross Unrealized Gains	779	1,140
Gross Unrealized Losses	(81)	(6)
Fair Value	25,317	23,634
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	199
Unrealized Losses Recorded in AOCI	0
Carrying Value	199
Gross Unrealized Gains	0
Gross Unrealized Losses	(1)
Fair Value	198
Agency | RMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	22,561	21,347
Unrealized Losses Recorded in AOCI	(1,048)	(1,184)
Carrying Value	21,513	20,163
Gross Unrealized Gains	692	1,047
Gross Unrealized Losses	(72)	0
Fair Value	22,133	21,210
Agency | CMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,012	2,457
Unrealized Losses Recorded in AOCI	(105)	(120)
Carrying Value	2,907	2,337
Gross Unrealized Gains	87	93
Gross Unrealized Losses	(8)	(6)
Fair Value	$ 2,986	$ 2,424