Investment Securities (Fair Value and Gross Unrealized Losses on Available-for-Sale Debt Securities) (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 4,128	$ 6,455
Less than 12 months Unrealized Losses	(34)	(70)
12 months or more Fair Value	383	15,111
12 months or more Unrealized Losses	(9)	(394)
Total Fair Value	4,511	21,566
Total Unrealized Losses	(43)	(464)
U.S. Treasury and federal agencies
Investments, Unrealized Loss Position
Less than 12 months Fair Value		0
Less than 12 months Unrealized Losses		0
12 months or more Fair Value		97
12 months or more Unrealized Losses		(1)
Total Fair Value		97
Total Unrealized Losses	0	(1)
Agency mortgage-backed securities | Residential mortgage backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	2,159	3,235
Less than 12 months Unrealized Losses	(19)	(21)
12 months or more Fair Value	4	7,892
12 months or more Unrealized Losses	0	(221)
Total Fair Value	2,163	11,127
Total Unrealized Losses	(19)	(242)
Agency mortgage-backed securities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	1,602	2,022
Less than 12 months Unrealized Losses	(12)	(37)
12 months or more Fair Value	0	5,260
12 months or more Unrealized Losses	0	(127)
Total Fair Value	1,602	7,282
Total Unrealized Losses	(12)	(164)
Non-agency mortgage-backed securities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value		884
Less than 12 months Unrealized Losses		(6)
12 months or more Fair Value		1,621
12 months or more Unrealized Losses		(41)
Total Fair Value		2,505
Total Unrealized Losses	0	(47)
Asset-backed securities and other debt securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	367	314
Less than 12 months Unrealized Losses	(3)	(6)
12 months or more Fair Value	379	241
12 months or more Unrealized Losses	(9)	(4)
Total Fair Value	746	555
Total Unrealized Losses	$ (12)	$ (10)