Investment Securities - Investment Securities Held to Maturity (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 24,901	$ 23,804
Unrealized Losses Recorded in AOCI	(1,190)	(1,304)
Carrying Value	23,711	22,500
Gross Unrealized Gains	1,220	1,140
Gross Unrealized Losses	(18)	(6)
Fair Value	24,913	23,634
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	198
Unrealized Losses Recorded in AOCI	0
Carrying Value	198
Gross Unrealized Gains	2
Gross Unrealized Losses	0
Fair Value	200
Agency | RMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	21,696	21,347
Unrealized Losses Recorded in AOCI	(1,082)	(1,184)
Carrying Value	20,614	20,163
Gross Unrealized Gains	1,078	1,047
Gross Unrealized Losses	(18)	0
Fair Value	21,674	21,210
Agency | CMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,007	2,457
Unrealized Losses Recorded in AOCI	(108)	(120)
Carrying Value	2,899	2,337
Gross Unrealized Gains	140	93
Gross Unrealized Losses	0	(6)
Fair Value	$ 3,039	$ 2,424