Loans Held for Investment and Loans Held for Investment at Fair Value - Summary of Loans Held for Investment (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounts Notes And Loans Receivable [Line Items]
Allowance for loan losses	$ (5,220)	$ (3,496)	$ (2,240)	$ (2,096)	$ (1,916)	$ (1,680)
Net Loans Held For Investment
Accounts Notes And Loans Receivable [Line Items]
Unpaid principal balance	1,841,238
Deferred loan origination costs	25,801
Total loans held for investment and loans held for investment at fair value, gross	1,867,039
Allowance for loan losses	(5,220)
Total loans held for investment and loans held for investment at fair value, net	1,861,819
Loans At Fair Value Held For Investment
Accounts Notes And Loans Receivable [Line Items]
Unpaid principal balance	3,369
Valuation adjustments on FVO loans	(413)
Total loans held for investment and loans held for investment at fair value, gross	2,956
Total loans held for investment and loans held for investment at fair value, net	2,956
Total Loans Held For Investment
Accounts Notes And Loans Receivable [Line Items]
Unpaid principal balance	1,844,607		1,843,290
Valuation adjustments on FVO loans	(413)		(444)
Deferred loan origination costs	25,801		25,714
Total loans held for investment and loans held for investment at fair value, gross	1,869,995		1,868,560
Allowance for loan losses	(5,220)		(2,240)
Total loans held for investment and loans held for investment at fair value, net	$ 1,864,775		$ 1,866,320