Investment Securities - Schedule of Available-for-Sale Securities in Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 11,614	$ 21,141
Less than 12 Months, Gross Unrealized Losses	(85)	(419)
12 Months or Longer, Fair Value	14,516	5,905
12 Months or Longer, Gross Unrealized Losses	(373)	(116)
Total Fair Value	26,130	27,046
Total Gross Unrealized Losses	(458)	(535)
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	2,031	1,060
Less than 12 Months, Gross Unrealized Losses	(8)	(49)
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Gross Unrealized Losses	0	0
Total Fair Value	2,031	1,060
Total Gross Unrealized Losses	(8)	(49)
RMBS
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	8,202	17,027
Less than 12 Months, Gross Unrealized Losses	(67)	(331)
12 Months or Longer, Fair Value	13,185	5,010
12 Months or Longer, Gross Unrealized Losses	(336)	(90)
Total Fair Value	21,387	22,037
Total Gross Unrealized Losses	(403)	(421)
RMBS | Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	8,192	16,899
Less than 12 Months, Gross Unrealized Losses	(67)	(329)
12 Months or Longer, Fair Value	13,175	4,865
12 Months or Longer, Gross Unrealized Losses	(335)	(83)
Total Fair Value	21,367	21,764
Total Gross Unrealized Losses	(402)	(412)
RMBS | Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	10	128
Less than 12 Months, Gross Unrealized Losses	0	(2)
12 Months or Longer, Fair Value	10	145
12 Months or Longer, Gross Unrealized Losses	(1)	(7)
Total Fair Value	20	273
Total Gross Unrealized Losses	(1)	(9)
CMBS
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		2,450
Less than 12 Months, Gross Unrealized Losses		(32)
12 Months or Longer, Fair Value		874
12 Months or Longer, Gross Unrealized Losses		(26)
Total Fair Value		3,324
Total Gross Unrealized Losses		(58)
CMBS | Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	880	1,624
Less than 12 Months, Gross Unrealized Losses	(8)	(21)
12 Months or Longer, Fair Value	1,236	745
12 Months or Longer, Gross Unrealized Losses	(36)	(24)
Total Fair Value	2,116	2,369
Total Gross Unrealized Losses	(44)	(45)
CMBS | Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		826
Less than 12 Months, Gross Unrealized Losses		(11)
12 Months or Longer, Fair Value		129
12 Months or Longer, Gross Unrealized Losses		(2)
Total Fair Value		955
Total Gross Unrealized Losses		(13)
Other ABS
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	130	187
Less than 12 Months, Gross Unrealized Losses	0	(1)
12 Months or Longer, Fair Value	95	21
12 Months or Longer, Gross Unrealized Losses	(1)	0
Total Fair Value	225	208
Total Gross Unrealized Losses	(1)	(1)
Other securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	371	417
Less than 12 Months, Gross Unrealized Losses	(2)	(6)
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Gross Unrealized Losses	0	0
Total Fair Value	371	417
Total Gross Unrealized Losses	$ (2)	$ (6)