Investment Securities (Gross Unrealized Loss and Fair Value of Securities Available for Sale Without an Allowance for Credit Losses) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	$ (455)	$ (42)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	34,446	5,185
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(22)	(27)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	2,014	1,724
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(477)	(69)
Debt securities, available-for-sale, unrealized loss position	36,460	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, accumulated loss	(266)	(13)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	12,798	603
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(266)	(13)
Debt securities, available-for-sale, unrealized loss position	12,798	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, accumulated loss	(170)	(8)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	19,019	3,152
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(2)	(2)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	75	82
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(172)	(10)
Debt securities, available-for-sale, unrealized loss position	19,094	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, accumulated 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, accumulated loss	(9)	(7)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	165	73
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(9)	(14)
Debt securities, available-for-sale, unrealized loss position	165	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, accumulated loss	(2)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	190
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(1)	(2)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	69	149
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(3)	(2)
Debt securities, available-for-sale, unrealized loss position	259	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, accumulated loss	(1)	(13)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	275	972
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(5)	(7)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	911	714
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(6)	(20)
Debt securities, available-for-sale, unrealized loss position	1,186	1,686
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(12)	(1)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	1,707	339
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(5)	(9)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	794	706
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(17)	(10)
Debt securities, available-for-sale, unrealized loss position	2,501	$ 1,045
Other
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(4)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	457
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(4)
Debt securities, available-for-sale, unrealized loss position	$ 457