Investment Securities (Fair Value and Gross Unrealized Losses on Available-for-Sale Debt Securities) (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 2,475	$ 4,128
Less than 12 months Unrealized Losses	(95)	(34)
12 months or more Fair Value	375	383
12 months or more Unrealized Losses	(15)	(9)
Total Fair Value	2,850	4,511
Total Unrealized Losses	(110)	(43)
Agency mortgage-backed securities | Residential mortgage backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	581	2,159
Less than 12 months Unrealized Losses	(16)	(19)
12 months or more Fair Value	2	4
12 months or more Unrealized Losses	0	0
Total Fair Value	583	2,163
Total Unrealized Losses	(16)	(19)
Agency mortgage-backed securities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	319	1,602
Less than 12 months Unrealized Losses	(5)	(12)
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	319	1,602
Total Unrealized Losses	(5)	(12)
Non-agency mortgage-backed securities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	109
Less than 12 months Unrealized Losses	(1)
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	109
Total Unrealized Losses	(1)	0
Asset-backed securities and other debt securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	1,466	367
Less than 12 months Unrealized Losses	(73)	(3)
12 months or more Fair Value	373	379
12 months or more Unrealized Losses	(15)	(9)
Total Fair Value	1,839	746
Total Unrealized Losses	$ (88)	$ (12)