Loans Held for Investment - Summary of Loan Portfolio Activity (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Loans And Leases Receivable Disclosure [Abstract]
Balance at December 31, 2018	$ 4,293,787
Loans originated	628,460
Additional fundings	57,394
Amortization of discount and origination fees	3,628
Collection of principal	(271,135)
Balance at March 31, 2019	$ 4,712,134