Investment Securities - Schedule of Available-for-Sale Securities in Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 6,594	$ 7,382
Less than 12 Months, Gross Unrealized Losses	(36)	(31)
12 Months or Longer, Fair Value	5,909	8,871
12 Months or Longer, Gross Unrealized Losses	(94)	(206)
Total Fair Value	12,503	16,253
Total Gross Unrealized Losses	(130)	(237)
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		1,499
Less than 12 Months, Gross Unrealized Losses		(1)
12 Months or Longer, Fair Value		0
12 Months or Longer, Gross Unrealized Losses		0
Total Fair Value		1,499
Total Gross Unrealized Losses		(1)
Corporate debt securities guaranteed by U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	113
Less than 12 Months, Gross Unrealized Losses	0	(2)
12 Months or Longer, Fair Value	249	557
12 Months or Longer, Gross Unrealized Losses	(2)	(18)
Total Fair Value	249	670
Total Gross Unrealized Losses	(2)	(20)
RMBS
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	5,438	4,329
Less than 12 Months, Gross Unrealized Losses	(33)	(24)
12 Months or Longer, Fair Value	3,917	4,503
12 Months or Longer, Gross Unrealized Losses	(57)	(94)
Total Fair Value	9,355	8,832
Total Gross Unrealized Losses	(90)	(118)
RMBS | Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	5,112	3,917
Less than 12 Months, Gross Unrealized Losses	(25)	(15)
12 Months or Longer, Fair Value	3,765	4,413
12 Months or Longer, Gross Unrealized Losses	(47)	(90)
Total Fair Value	8,877	8,330
Total Gross Unrealized Losses	(72)	(105)
RMBS | Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	326	412
Less than 12 Months, Gross Unrealized Losses	(8)	(9)
12 Months or Longer, Fair Value	152	90
12 Months or Longer, Gross Unrealized Losses	(10)	(4)
Total Fair Value	478	502
Total Gross Unrealized Losses	(18)	(13)
CMBS
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	700	552
Less than 12 Months, Gross Unrealized Losses	(3)	(3)
12 Months or Longer, Fair Value	1,557	2,674
12 Months or Longer, Gross Unrealized Losses	(33)	(72)
Total Fair Value	2,257	3,226
Total Gross Unrealized Losses	(36)	(75)
CMBS | Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	265	294
Less than 12 Months, Gross Unrealized Losses	(1)	(2)
12 Months or Longer, Fair Value	1,230	1,993
12 Months or Longer, Gross Unrealized Losses	(29)	(58)
Total Fair Value	1,495	2,287
Total Gross Unrealized Losses	(30)	(60)
CMBS | Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	435	258
Less than 12 Months, Gross Unrealized Losses	(2)	(1)
12 Months or Longer, Fair Value	327	681
12 Months or Longer, Gross Unrealized Losses	(4)	(14)
Total Fair Value	762	939
Total Gross Unrealized Losses	(6)	(15)
Other ABS
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	400	783
Less than 12 Months, Gross Unrealized Losses	0	(1)
12 Months or Longer, Fair Value	166	586
12 Months or Longer, Gross Unrealized Losses	(1)	(9)
Total Fair Value	566	1,369
Total Gross Unrealized Losses	(1)	(10)
Other securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	56	106
Less than 12 Months, Gross Unrealized Losses	0	0
12 Months or Longer, Fair Value	20	551
12 Months or Longer, Gross Unrealized Losses	(1)	(13)
Total Fair Value	76	657
Total Gross Unrealized Losses	$ (1)	$ (13)