Investment Securities Investment Securities (Gross Unrealized Loss and Fair Value of Securities Available for Sale Without an Allowance for Credit Losses) (Details) $ in Millions	Sep. 30, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	$ (62)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	5,696
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(29)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	1,284
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(91)
Debt securities, available-for-sale, unrealized loss position	6,980
US Treasury and Government [Member]
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	167
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(2)
Debt securities, available-for-sale, unrealized loss position	167
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	(9)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	2,860
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	102
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(11)
Debt securities, available-for-sale, unrealized loss position	2,962
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	(9)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	160
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	(16)
Debt securities, available-for-sale, unrealized loss position	233
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, 12 months or longer, accumulated loss	(2)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	132
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(2)
Debt securities, available-for-sale, unrealized loss position	132
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	(34)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	1,638
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	236
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(36)
Debt securities, available-for-sale, unrealized loss position	1,874
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	(7)
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months - fair value	844
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(16)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer - fair value	741
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(23)
Debt securities, available-for-sale, unrealized loss position	1,585
Other [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	27
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(1)
Debt securities, available-for-sale, unrealized loss position	$ 27

Investment Securities Investment Securities (Gross Unrealized Loss and Fair Value of Securities Available for Sale Without an Allowance for Credit Losses) (Details)
$ in Millions