Credit Enhancement Receivable, Credit Enhancement Deposit and Contingent Liability - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit
Credit enhancement receivable	$ 0	$ 0
Deposit in Fannie Mae restricted liquidity account	25,000,000.0	25,000,000.0
DB Cayman | Credit Risk
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit
Reimbursements under serving agreement	$ 0	0
Percentage of contingent payment (in percentage)	50.00%
Contingent liability	$ 12,300,000	$ 11,800,000