Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle
Guaranty liabilities	$ 29,581	$ 15	$ 32
Provision for expected credit losses	11,631
Total stockholders’ equity	$ 655,049	599,664
Allowance for doubtful accounts		11,800
Quarterly net profit allocation (percent)	0.6875%
Annualized net profit allocation (percent)	2.75%
Servicing Fees
New Accounting Pronouncements or Change in Accounting Principle
Interest rate (percent)	0.03%
Surveillance Fees
New Accounting Pronouncements or Change in Accounting Principle
Interest rate (percent)	0.01%
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Guaranty liabilities		17,935
Loans Receivable
New Accounting Pronouncements or Change in Accounting Principle
Allowance for credit loss	$ 3,700
Loans Receivable | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Total stockholders’ equity		$ 3,700