Related Party Transactions (Details) $ in Thousands, £ in Millions			1 Months Ended				3 Months Ended				9 Months Ended
		May 24, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Jan. 31, 2020 USD ($)	Jan. 31, 2020 GBP (£)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2020 GBP (£)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 29, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2017 USD ($)
Related Party Transaction [Line Items]
Related party expenses							$ 9,583	$ 9,927			$ 28,387	$ 30,152
Base management fees incurred but not yet paid							9,583				9,583			$ 9,598
Total carrying value, net							7,257,304				7,257,304			6,496,977
Subordinate Mortgage Portfolio Segment
Related Party Transaction [Line Items]
Total carrying value, net	[1]						817,759				817,759			$ 1,045,893
Senior Mezzanine Loans
Related Party Transaction [Line Items]
Total carrying value, net							234,300				$ 234,300
Hotel - Washington D.C. | Subordinate Mortgage Portfolio Segment
Related Party Transaction [Line Items]
Debt related to real estate owned, held for investment		$ 110,000											$ 110,000		$ 110,000
Hotel - Washington D.C. | Junior Mezzanine Loans
Related Party Transaction [Line Items]
Total carrying value, net															20,000
Hotel - Washington D.C. | Senior Mezzanine Loans
Related Party Transaction [Line Items]
Total carrying value, net															$ 24,500
Payments to acquire mortgage notes receivable		$ 24,500
Sale of Mezzanine Loan
Related Party Transaction [Line Items]
Proceeds from sale of loan					$ 81,300	£ 62.2			$ 81,300	£ 62.2
Sale of Unfunded Commitment of Senior Mortgage
Related Party Transaction [Line Items]
Proceeds from sale of loan					$ 65,300	£ 50.0			$ 65,300	£ 50.0
Arrangement fees
Related Party Transaction [Line Items]
Related party expenses			$ 400	$ 200
Limited Liability Company
Related Party Transaction [Line Items]
Rate of management fees											1.50%
Extension period											1 year
Period of termination											180 days
Termination fee calculation period											24 months
Limited Liability Company | Management fees
Related Party Transaction [Line Items]
Base management fees incurred but not yet paid							9,600				$ 9,600
Affiliated Entity
Related Party Transaction [Line Items]
Debt instrument, face amount				$ 30,000
Affiliated Entity | Management fees
Related Party Transaction [Line Items]
Related party expenses							9,600	9,900			28,400	30,200
Affiliated Entity | Reimbursements
Related Party Transaction [Line Items]
Related party expenses							$ 500	$ 600			$ 1,700	$ 1,700

[1] Net of $177,684 and $213,102 CECL Allowances in 2021 and 2020, respectively, comprised of $145,000 and $175,000 Specific CECL Allowance and $32,684 and $38,102 General CECL Allowance, respectively.