Investment Securities (Fair Value and Gross Unrealized Losses on Available-for-Sale Debt Securities) (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 6,455	$ 11,101
Less than 12 months Unrealized Losses	(70)	(85)
12 months or more Fair Value	15,111	1,536
12 months or more Unrealized Losses	(394)	(49)
Total Fair Value	21,566	12,637
Total Unrealized Losses	(464)	(134)
U.S. Treasury and federal agencies
Investments, Unrealized Loss Position
Less than 12 months Fair Value	0	98
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	97	0
12 months or more Unrealized Losses	(1)	0
Total Fair Value	97	98
Total Unrealized Losses	(1)	0
Agency mortgage-backed securities | Residential mortgage backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	3,235	7,337
Less than 12 months Unrealized Losses	(21)	(59)
12 months or more Fair Value	7,892	479
12 months or more Unrealized Losses	(221)	(21)
Total Fair Value	11,127	7,816
Total Unrealized Losses	(242)	(80)
Agency mortgage-backed securities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	2,022	2,900
Less than 12 months Unrealized Losses	(37)	(22)
12 months or more Fair Value	5,260	526
12 months or more Unrealized Losses	(127)	(16)
Total Fair Value	7,282	3,426
Total Unrealized Losses	(164)	(38)
Non-agency mortgage-backed securities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	884	449
Less than 12 months Unrealized Losses	(6)	(2)
12 months or more Fair Value	1,621	145
12 months or more Unrealized Losses	(41)	(3)
Total Fair Value	2,505	594
Total Unrealized Losses	(47)	(5)
Asset-backed securities and other debt securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	314	317
Less than 12 months Unrealized Losses	(6)	(2)
12 months or more Fair Value	241	386
12 months or more Unrealized Losses	(4)	(9)
Total Fair Value	555	703
Total Unrealized Losses	$ (10)	$ (11)