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Loans Held-for-Investment - Summary of TDRs by Type and Performing Status and Newly Modified TDRs (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($) loan	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($) loan	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Troubled Debt Restructurings
TDRs	$ 48		$ 48			$ 46
Allowance for loan loss	171	$ 255	171	$ 255	$ 202	252	$ 229	$ 107
TDR loans under fair value option	$ 1		$ 1			3
Number of Accounts (in loans) \| loan	22	3	35	12
Pre-Modification Unpaid Principal Balance	$ 8	$ 0	$ 16	$ 6
Post-Modification Unpaid Principal Balance	8	0	16	6
Troubled debt restructurings
Troubled Debt Restructurings
Allowance for loan loss	4		4			5
Performing
Troubled Debt Restructurings
TDRs	34		34			36
Nonperforming
Troubled Debt Restructurings
TDRs	14		14			10
Consumer loans
Troubled Debt Restructurings
TDRs	46		46			41
Consumer loans \| Performing
Troubled Debt Restructurings
TDRs	34		34			31
Consumer loans \| Nonperforming
Troubled Debt Restructurings
TDRs	12		12			10
Consumer loans \| Residential first mortgage loans
Troubled Debt Restructurings
TDRs	34		34			27
Allowance for loan loss	$ 43	$ 52	$ 43	$ 52	48	49	60	22
Number of Accounts (in loans) \| loan	21	1	32	6
Pre-Modification Unpaid Principal Balance	$ 8	$ 0	$ 14	$ 1
Post-Modification Unpaid Principal Balance	8	0	14	1
Consumer loans \| Residential first mortgage loans \| Performing
Troubled Debt Restructurings
TDRs	25		25			19
Consumer loans \| Residential first mortgage loans \| Nonperforming
Troubled Debt Restructurings
TDRs	9		9			8
Consumer loans \| Home equity
Troubled Debt Restructurings
TDRs	12		12			14
Allowance for loan loss	$ 15	$ 29	$ 15	$ 29	17	25	28	14
Number of Accounts (in loans) \| loan	1	1	2	3
Pre-Modification Unpaid Principal Balance	$ 0	$ 0	$ 0	$ 0
Post-Modification Unpaid Principal Balance	0	$ 0	0	$ 0
Consumer loans \| Home equity \| Performing
Troubled Debt Restructurings
TDRs	9		9			12
Consumer loans \| Home equity \| Nonperforming
Troubled Debt Restructurings
TDRs	3		$ 3			2
Consumer loans \| Consumer
Troubled Debt Restructurings
Number of Accounts (in loans) \| loan		1	0	2
Pre-Modification Unpaid Principal Balance		$ 0	$ 0	$ 0
Post-Modification Unpaid Principal Balance		0	0	$ 0
Commercial loans
Troubled Debt Restructurings
TDRs						5
Commercial loans \| Performing
Troubled Debt Restructurings
TDRs						5
Commercial loans \| Nonperforming
Troubled Debt Restructurings
TDRs						0
Commercial loans \| Commercial real estate
Troubled Debt Restructurings
TDRs	$ 0		$ 0
Number of Accounts (in loans) \| loan	0		1	1
Pre-Modification Unpaid Principal Balance	$ 0		$ 2	$ 5
Post-Modification Unpaid Principal Balance	0		2	5
Commercial loans \| Commercial real estate \| Performing
Troubled Debt Restructurings
TDRs	0		0
Commercial loans \| Commercial real estate \| Nonperforming
Troubled Debt Restructurings
TDRs	0		0
Commercial loans \| Commercial and industrial
Troubled Debt Restructurings
TDRs	2		2
Allowance for loan loss	43	$ 42	43	$ 42	$ 38	$ 51	$ 23	$ 22
Commercial loans \| Commercial and industrial \| Performing
Troubled Debt Restructurings
TDRs	0		0
Commercial loans \| Commercial and industrial \| Nonperforming
Troubled Debt Restructurings
TDRs	$ 2		$ 2