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Loans Held-for-Investment - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Modifications subsequent to default (less than) $ 0 $ 0 $ 1,000,000
Interest lost on nonaccrual loans 1,000,000 1,000,000 $ 1,000,000
Loans 90 days or greater past due and still accruing 0 0  
Loans held-for-investment      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans pledged as collateral $ 9,100,000,000 $ 6,800,000,000