Rollforward of Loans Held-for-Investment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Receivables [Abstract]
Loans held-for-investment pledged as collateral for borrowings	$ 3,900,000		$ 3,900,000		$ 4,100,000
Loans Held-for-Investment [Roll Forward]
Balance at beginning of period	4,290,047	$ 3,560,117	4,226,212	$ 3,167,913
Originations, acquisitions and additional fundings	57,617	535,021	317,630	1,230,712
Repayments	(184,723)	(164,797)	(290,838)	(468,534)
Transfers to loans held-for-sale	(190,787)	0	(210,452)	0
Net discount accretion (premium amortization)	0	4	16	24
Increase in net deferred origination fees	101	(6,387)	(2,908)	(14,034)
Amortization of net deferred origination fees	3,236	3,137	12,541	11,014
Allowance for credit losses	3,371	0	(73,339)	0
Balance at end of period	$ 3,978,862	$ 3,927,095	$ 3,978,862	$ 3,927,095