Loans Held for Investment and the Allowance for Credit Losses - Summary of Loans Held for Investment Portfolio Activity (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($)
Loans And Leases Receivable Disclosure [Abstract]
Balance as of December 31, 2020	$ 4,456,460
Loans originated and acquired	1,062,872
Additional fundings	111,583
Amortization of origination fees	5,448
Collection of principal	(799,688)
Loans held for sale	(86,636)
Loan sale	(60,690)
Change in allowance for credit losses	9,889
Balance as of September 30, 2021	$ 4,699,238