Investment Securities - Schedule of Available-for-Sale Securities in Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 Months	$ 5,118	$ 12,743
12 Months or Longer	25,951	21,422
Total	31,069	34,165
Gross Unrealized Losses
Less than 12 Months	(17)	(297)
12 Months or Longer	(634)	(675)
Total	(651)	(972)
U.S. Treasury securities
Fair Value
Less than 12 Months	2,583	2,543
12 Months or Longer	1,085	1,076
Total	3,668	3,619
Gross Unrealized Losses
Less than 12 Months	(5)	(3)
12 Months or Longer	(5)	(14)
Total	(10)	(17)
RMBS, Agency
Fair Value
Less than 12 Months	1,666	7,863
12 Months or Longer	21,986	18,118
Total	23,652	25,981
Gross Unrealized Losses
Less than 12 Months	(8)	(260)
12 Months or Longer	(579)	(609)
Total	(587)	(869)
RMBS, Non-agency
Fair Value
Less than 12 Months	61	89
12 Months or Longer	9	10
Total	70	99
Gross Unrealized Losses
Less than 12 Months	(1)	(2)
12 Months or Longer	(1)	0
Total	(2)	(2)
RMBS
Fair Value
Less than 12 Months	1,727	7,952
12 Months or Longer	21,995	18,128
Total	23,722	26,080
Gross Unrealized Losses
Less than 12 Months	(9)	(262)
12 Months or Longer	(580)	(609)
Total	(589)	(871)
CMBS, Agency
Fair Value
Less than 12 Months	499	2,004
12 Months or Longer	2,396	1,540
Total	2,895	3,544
Gross Unrealized Losses
Less than 12 Months	(2)	(31)
12 Months or Longer	(46)	(47)
Total	(48)	(78)
Other securities
Fair Value
Less than 12 Months	309	244
12 Months or Longer	475	678
Total	784	922
Gross Unrealized Losses
Less than 12 Months	(1)	(1)
12 Months or Longer	(3)	(5)
Total	$ (4)	$ (6)