Investment Securities Investment Securities (Gross Unrealized Loss and Fair Value of Securities Available for Sale Without an Allowance for Credit Losses) (Details) $ in Millions	Mar. 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	$ (297)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	6,987
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(76)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	1,578
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(373)
Debt securities, available-for-sale, unrealized loss position	8,565
Residential Mortgage-backed Securities [Member] | Mortgage-backed Securities Agency [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(3)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	409
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	380
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(10)
Debt securities, available-for-sale, unrealized loss position	789
Residential Mortgage-backed Securities [Member] | Mortgage-backed Securities Non-agency [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(43)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	537
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(17)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	77
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(60)
Debt securities, available-for-sale, unrealized loss position	614
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities Agency [Member]
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	187
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(5)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	436
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(6)
Debt securities, available-for-sale, unrealized loss position	623
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities Non-agency [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(172)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	2,719
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(22)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	147
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(194)
Debt securities, available-for-sale, unrealized loss position	2,866
Asset backed [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(75)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	2,815
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(25)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	538
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(100)
Debt securities, available-for-sale, unrealized loss position	3,353
Other [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(3)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	320
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(3)
Debt securities, available-for-sale, unrealized loss position	$ 320