Commercial Mortgage Loans - Commercial Mortgage Loan Portfolio, Held-For-Investment (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Loan loss increase/(decrease)	$ (14,597)	$ (2,495)
Commercial Mortgage Receivable, Held-For-Investment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance at Beginning of Year	2,762,042	2,206,830	$ 2,206,830
Acquisitions and originations	288,825		1,326,983
Principal repayments	(366,058)		(771,774)
Discount accretion/premium amortization	1,751		6,264
Loans reclassified to held-for-sale	(9,619)		0
Loans transferred from/(to) commercial real estate loans, held-for-sale	0		10,100
Net fees capitalized into carrying value of loans	(1,181)		(5,339)
Loan loss increase/(decrease)	(13,997)		(3,007)
Loan loss increase/(decrease)	427		6,922
Transfer on foreclosure to real estate owned	(14,000)		0
Transfer on deed in lieu of foreclosure to real estate owned	0		(14,937)
Balance at End of Period	2,640,979		2,762,042
Commercial Mortgage Receivable, Held-For-Investment | Transition Adjustment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance at Beginning of Year	$ (7,211)	$ 0	0
Balance at End of Period			$ (7,211)