Loans Held-for-Investment - Narrative (Details)	3 Months Ended
	Jun. 30, 2021 USD ($) loans	Dec. 31, 2020 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other assets	$ 1,168,000,000	$ 1,250,000,000
Loans pledged as collateral	10,900,000,000	11,600,000,000
Allowance for loan loss	202,000,000	252,000,000	$ 241,000,000	$ 229,000,000	$ 132,000,000	$ 107,000,000
Interest income on nonaccrual loans using cash basis method	1,000,000	2,000,000
Loans 90 days past due and still accruing interest	0	0
Other liabilities	$ 1,226,000,000	1,272,000,000
Number of loans modified in previous 12 months that have defaulted | loans	0
Unfunded Commitments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other liabilities	$ 18,000,000			$ 21,000,000
Accrued interest receivable on loans held-for-investment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other assets	$ 39,000,000	$ 43,000,000