Investment Securities (Gross Unrealized Loss and Fair Value of Securities Available for Sale Without an Allowance for Credit Losses) (Details) $ in Millions	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	$ (42)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	5,185
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(27)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	1,724
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(69)
Debt securities, available-for-sale, unrealized loss position	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	(13)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	603
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(13)
Debt securities, available-for-sale, unrealized loss position	603
Residential mortgage-backed Securities | 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	(8)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	3,152
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(2)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	82
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(10)
Debt securities, available-for-sale, unrealized loss position	3,234
Residential mortgage-backed Securities | 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	(7)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	73
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(14)
Debt securities, available-for-sale, unrealized loss position	192
Commercial Mortgage Backed Securities | Mortgage-backed Securities Agency
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(2)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	149
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(2)
Debt securities, available-for-sale, unrealized loss position	149
Commercial Mortgage Backed Securities | 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	(13)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	972
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(7)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	714
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(20)
Debt securities, available-for-sale, unrealized loss position	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	(1)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	339
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(9)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	706
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(10)
Debt securities, available-for-sale, unrealized loss position	$ 1,045