Investment Securities - Fair Value and Gross Unrealized Losses on Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair Value
Less than 12 months	$ 3,550	$ 1,336
12 months or more	832	604
Total	4,382	1,940
Unrealized Losses
Less than 12 months	(141)	(20)
12 months or more	(20)	(17)
Total	(161)	(37)
Obligations of states and political subdivisions securities
Fair Value
Less than 12 months	1
12 months or more	0
Total	1
Unrealized Losses
Less than 12 months	0
12 months or more	0
Total	0
Agency mortgage-backed securities | Residential mortgage backed securities
Fair Value
Less than 12 months	977	426
12 months or more	1	1
Total	978	427
Unrealized Losses
Less than 12 months	(35)	(8)
12 months or more	0	0
Total	(35)	(8)
Agency mortgage-backed securities | Commercial mortgage-backed securities
Fair Value
Less than 12 months	2,162	388
12 months or more	0	0
Total	2,162	388
Unrealized Losses
Less than 12 months	(97)	(5)
12 months or more	0	0
Total	(97)	(5)
Non-agency mortgage-backed securities | Commercial mortgage-backed securities
Fair Value
Less than 12 months	3	2
12 months or more	0	0
Total	3	2
Unrealized Losses
Less than 12 months	0	0
12 months or more	0	0
Total	0	0
Asset-backed securities and other debt securities
Fair Value
Less than 12 months	407	520
12 months or more	831	603
Total	1,238	1,123
Unrealized Losses
Less than 12 months	(9)	(7)
12 months or more	(20)	(17)
Total	$ (29)	$ (24)