Investment Securities - Investment Securities Held to Maturity (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 23,521	$ 20,567
Unrealized Losses Recorded in AOCI	(1,339)	(1,435)
Carrying Value	22,182	19,132
Gross Unrealized Gains	759	88
Gross Unrealized Losses	(13)	(35)
Fair Value	22,928	19,185
Agency | RMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	21,565	18,746
Unrealized Losses Recorded in AOCI	(1,216)	(1,303)
Carrying Value	20,349	17,443
Gross Unrealized Gains	694	72
Gross Unrealized Losses	(5)	(30)
Fair Value	21,038	17,485
Agency | CMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,956	1,821
Unrealized Losses Recorded in AOCI	(123)	(132)
Carrying Value	1,833	1,689
Gross Unrealized Gains	65	16
Gross Unrealized Losses	(8)	(5)
Fair Value	$ 1,890	$ 1,700