Commercial Mortgage, Subordinate Loans and Other Lending Assets, Net - Statistics for Loan Portfolio (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | loan	70	72
Principal balance	$ 6,476,313	$ 6,375,093
Carrying value, net	6,437,037	6,375,093
Unfunded loan commitments	$ 1,426,404	$ 1,952,887
Weighted-average cash coupon	5.60%	6.50%
Weighted-average remaining fully-extended term	2 years 10 months 24 days	3 years 3 months 18 days
Weighted-average expected term	2 years	1 year 9 months 18 days
Interest rate used in calculating weighted-average cash coupon for non-accrual or cost recovery loans	0.00%	0.00%
Principal balance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal balance	$ 6,694,144	$ 6,467,842