Investment Securities - Fair Value and Gross Unrealized Losses on Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value
Less than 12 months	$ 5,222	$ 1,336
12 months or more	577	604
Total	5,799	1,940
Unrealized Losses
Less than 12 months	(87)	(20)
12 months or more	(16)	(17)
Total	(103)	(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
U.S. Treasury and federal agencies securities
Fair Value
Less than 12 months	15
12 months or more	0
Total	15
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	844	426
12 months or more	91	1
Total	935	427
Unrealized Losses
Less than 12 months	(9)	(8)
12 months or more	(4)	0
Total	(13)	(8)
Agency mortgage-backed securities | Commercial mortgage-backed securities
Fair Value
Less than 12 months	2,801	388
12 months or more	24	0
Total	2,825	388
Unrealized Losses
Less than 12 months	(64)	(5)
12 months or more	(2)	0
Total	(66)	(5)
Non-agency mortgage-backed securities | Commercial mortgage-backed securities
Fair Value
Less than 12 months	515	2
12 months or more	0	0
Total	515	2
Unrealized Losses
Less than 12 months	(6)	0
12 months or more	0	0
Total	(6)	0
Asset-backed securities and other debt securities
Fair Value
Less than 12 months	1,046	520
12 months or more	462	603
Total	1,508	1,123
Unrealized Losses
Less than 12 months	(8)	(7)
12 months or more	(10)	(17)
Total	$ (18)	$ (24)