Loans Held-for-Investment - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) loans	Dec. 31, 2020 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other assets	$ 1,199	$ 1,250
Loans pledged as collateral	4,300	11,600
Allowance for loan loss	171	252	$ 202	$ 255	$ 229	$ 107
Interest income on nonaccrual loans using cash basis method	1	2
Loans 90 days past due and still accruing interest	0	0
Other liabilities	$ 1,232	1,272
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	$ 19	28
Accrued interest receivable on loans held-for-investment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other assets	$ 38	$ 43