Loans Held-for-Investment - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income that would have been accrued	$ 1,000,000	$ 1,000,000	$ 1,000,000
Loans 90 days or greater past due and still accruing	0	0
Collateral pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans pledged as collateral	80,000,000	54,000,000
Loans held-for-investment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans pledged as collateral	$ 11,600,000,000	$ 9,100,000,000