Investment Securities - Investment Securities Held to Maturity (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 26,200	$ 25,772
Unrealized Losses Recorded in AOCI	(1,120)	(1,153)
Carrying Value	25,080	24,619
Gross Unrealized Gains	1,381	779
Gross Unrealized Losses	(7)	(81)
Fair Value	26,454	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	1	0
Gross Unrealized Losses	0	(1)
Fair Value	200	198
Agency | RMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	22,846	22,561
Unrealized Losses Recorded in AOCI	(1,018)	(1,048)
Carrying Value	21,828	21,513
Gross Unrealized Gains	1,216	692
Gross Unrealized Losses	(6)	(72)
Fair Value	23,038	22,133
Agency | CMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,155	3,012
Unrealized Losses Recorded in AOCI	(102)	(105)
Carrying Value	3,053	2,907
Gross Unrealized Gains	164	87
Gross Unrealized Losses	(1)	(8)
Fair Value	$ 3,216	$ 2,986