Investment Securities (Gross Unrealized Loss and Fair Value of Securities Available for Sale Without an Allowance for Credit Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, unrealized loss	$ (320)	$ (42)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	56,419	5,185
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, unrealized loss	(14)	(27)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	2,152	1,724
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(334)	(69)
Debt securities, available-for-sale, unrealized loss position, fair value	58,571	6,909
U.S. Treasury and government agencies
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, unrealized loss	(131)	(13)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	21,970	603
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(131)	(13)
Debt securities, available-for-sale, unrealized loss position, fair value	21,970	603
Residential mortgage-backed | Mortgage-backed Securities Agency
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, unrealized loss	(177)	(8)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	31,410	3,152
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, unrealized loss	(7)	(2)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	1,135	82
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(184)	(10)
Debt securities, available-for-sale, unrealized loss position, fair value	32,545	3,234
Residential mortgage-backed | Mortgage-backed Securities Non-agency
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, unrealized loss		(7)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value		119
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, unrealized loss	(2)	(7)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	136	73
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(2)	(14)
Debt securities, available-for-sale, unrealized loss position, fair value	136	192
Commercial mortgage-backed | Mortgage-backed Securities Agency
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, unrealized loss	(3)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	380
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, unrealized loss	(1)	(2)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	61	149
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(4)	(2)
Debt securities, available-for-sale, unrealized loss position, fair value	441	149
Commercial mortgage-backed | Mortgage-backed Securities Non-agency
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, unrealized loss	(1)	(13)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	661	972
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, unrealized loss	(2)	(7)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	489	714
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(3)	(20)
Debt securities, available-for-sale, unrealized loss position, fair value	1,150	1,686
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, unrealized loss	(5)	(1)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	1,585	339
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, unrealized loss	(2)	(9)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	331	706
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(7)	(10)
Debt securities, available-for-sale, unrealized loss position, fair value	1,916	$ 1,045
Other
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, unrealized loss	(3)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	413
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(3)
Debt securities, available-for-sale, unrealized loss position, fair value	$ 413