Investment Securities - Schedule of Available-for-Sale Securities in Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 6,625	$ 18,642
Less than 12 Months, Gross Unrealized Losses	(44)	(156)
12 Months or Longer, Fair Value	5,534	6,328
12 Months or Longer, Gross Unrealized Losses	(85)	(165)
Total Fair Value	12,159	24,970
Total Gross Unrealized Losses	(129)	(321)
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	3,096
Less than 12 Months, Gross Unrealized Losses	0	(9)
12 Months or Longer, Fair Value	1	1
12 Months or Longer, Gross Unrealized Losses	0	0
Total Fair Value	1	3,097
Total Gross Unrealized Losses	0	(9)
RMBS
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	5,116	12,380
Less than 12 Months, Gross Unrealized Losses	(35)	(120)
12 Months or Longer, Fair Value	4,181	4,575
12 Months or Longer, Gross Unrealized Losses	(59)	(114)
Total Fair Value	9,297	16,955
Total Gross Unrealized Losses	(94)	(234)
RMBS | Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	4,679	12,025
Less than 12 Months, Gross Unrealized Losses	(22)	(110)
12 Months or Longer, Fair Value	4,019	4,420
12 Months or Longer, Gross Unrealized Losses	(46)	(102)
Total Fair Value	8,698	16,445
Total Gross Unrealized Losses	(68)	(212)
RMBS | Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	437	355
Less than 12 Months, Gross Unrealized Losses	(13)	(10)
12 Months or Longer, Fair Value	162	155
12 Months or Longer, Gross Unrealized Losses	(13)	(12)
Total Fair Value	599	510
Total Gross Unrealized Losses	(26)	(22)
CMBS
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	931	2,091
Less than 12 Months, Gross Unrealized Losses	(5)	(22)
12 Months or Longer, Fair Value	1,156	1,478
12 Months or Longer, Gross Unrealized Losses	(25)	(49)
Total Fair Value	2,087	3,569
Total Gross Unrealized Losses	(30)	(71)
CMBS | Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	590	1,352
Less than 12 Months, Gross Unrealized Losses	(3)	(9)
12 Months or Longer, Fair Value	927	1,148
12 Months or Longer, Gross Unrealized Losses	(22)	(38)
Total Fair Value	1,517	2,500
Total Gross Unrealized Losses	(25)	(47)
CMBS | Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	341	739
Less than 12 Months, Gross Unrealized Losses	(2)	(13)
12 Months or Longer, Fair Value	229	330
12 Months or Longer, Gross Unrealized Losses	(3)	(11)
Total Fair Value	570	1,069
Total Gross Unrealized Losses	(5)	(24)
Other ABS
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	368	825
Less than 12 Months, Gross Unrealized Losses	(1)	(5)
12 Months or Longer, Fair Value	177	255
12 Months or Longer, Gross Unrealized Losses	0	(1)
Total Fair Value	545	1,080
Total Gross Unrealized Losses	(1)	(6)
Other securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	210	250
Less than 12 Months, Gross Unrealized Losses	(3)	0
12 Months or Longer, Fair Value	19	19
12 Months or Longer, Gross Unrealized Losses	(1)	(1)
Total Fair Value	229	269
Total Gross Unrealized Losses	$ (4)	$ (1)