Loans Portfolio - Schedule of Loans and Financing Receivable (Details) $ in Thousands		9 Months Ended		12 Months Ended
		Sep. 30, 2021 USD ($) Investment Loan		Dec. 31, 2020 USD ($) Investment Loan		Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Financing Receivable Recorded Investment Past Due [Line Items]
Loan Commitment(5)	[1]	$ 6,970,592		$ 7,349,651
Principal Outstanding		6,009,765		6,152,331
Loans receivable held-for-investment		$ 5,983,559		$ 6,131,825
Number of Loans | Loan		97		98
Weighted Average Interest Rate(4)		6.10%	[2]	6.57%	[3]
Less: allowance for loan losses		$ (59,213)		$ (6,000)
Loans receivable held-for-investment, net		5,924,346		6,125,825	[4]	$ 6,180,166	$ 5,940,268
Variable Senior Loans
Financing Receivable Recorded Investment Past Due [Line Items]
Loan Commitment(5)	[1],[6]	6,452,082	[5]	6,395,473	[7]
Principal Outstanding	[6]	5,515,249	[5]	5,272,201	[7]
Loans receivable held-for-investment	[6]	$ 5,487,984	[5]	$ 5,252,577	[4],[7]
Variable Senior Loans | London Interbank Offered Rate (LIBOR)
Financing Receivable Recorded Investment Past Due [Line Items]
Number of Investments | Investment	[6]	48	[5]	46	[7]
Number of Loans | Loan	[6]	84	[5]	80	[7]
Weighted Average Stated Rate(2)	[6]	L + 4.29%	[5],[8]	L + 4.35%	[7],[9]
Weighted Average Interest Rate(4)	[6]	5.68%	[2],[5]	5.93%	[3],[7]
Variable Subordinate Loans
Financing Receivable Recorded Investment Past Due [Line Items]
Loan Commitment(5)	[1]	$ 436,544		$ 852,982
Principal Outstanding		413,967		782,646
Loans receivable held-for-investment		$ 415,056		$ 782,167	[4]
Variable Subordinate Loans | London Interbank Offered Rate (LIBOR)
Financing Receivable Recorded Investment Past Due [Line Items]
Number of Investments | Investment		4		8
Number of Loans | Loan		6		11
Weighted Average Stated Rate(2)		L + 8.95%	[8]	L + 8.72%	[9]
Weighted Average Interest Rate(4)		10.71%	[2]	10.20%	[3]
Variable Loan
Financing Receivable Recorded Investment Past Due [Line Items]
Loan Commitment(5)	[1]	$ 6,888,626		$ 7,248,455
Principal Outstanding		5,929,216		6,054,847
Loans receivable held-for-investment		$ 5,903,040		$ 6,034,744	[4]
Variable Loan | London Interbank Offered Rate (LIBOR)
Financing Receivable Recorded Investment Past Due [Line Items]
Weighted Average Stated Rate(2)		L + 4.62%	[8]	L + 4.91%	[9]
Weighted Average Interest Rate(4)		6.03%	[2]	6.48%	[3]
Fixed Senior Loans
Financing Receivable Recorded Investment Past Due [Line Items]
Loan Commitment(5)	[1],[6]	$ 58,939	[5]	$ 75,669	[7]
Principal Outstanding	[6]	58,939	[5]	75,669	[7]
Loans receivable held-for-investment	[6]	$ 59,073	[5]	$ 69,523	[4],[7]
Number of Investments | Investment	[6]	2	[5]	2	[7]
Number of Loans | Loan	[6]	3	[5]	3	[7]
Weighted Average Interest Rate(4)	[6]	11.27%	[2],[5]	12.59%	[3],[7]
Fixed Subordinate Loans
Financing Receivable Recorded Investment Past Due [Line Items]
Loan Commitment(5)	[1]	$ 23,027		$ 25,527
Principal Outstanding		21,610		21,815
Loans receivable held-for-investment		$ 21,446		$ 21,558	[4]
Number of Investments | Investment		2		2
Number of Loans | Loan		2		2
Weighted Average Interest Rate(4)		11.55%	[2]	11.00%	[3]
Fixed Loan
Financing Receivable Recorded Investment Past Due [Line Items]
Loan Commitment(5)	[1]	$ 81,966		$ 101,196
Principal Outstanding		80,549		97,484
Loans receivable held-for-investment		$ 80,519		$ 91,081	[4]
Weighted Average Interest Rate(4)		11.35%	[2]	12.24%	[3]

[1] Loan commitment represents initial loan commitments, as adjusted by commitment reductions, less loan repayments and transfers which qualified for sale accounting under GAAP

[2] Reflects the weighted average interest rate based on the applicable floating benchmark rate (if applicable), including LIBOR floors (if applicable). Weighted average is based on outstanding principal as of September 30, 2021.

[3] Reflects the weighted average interest rate based on the applicable floating benchmark rate (if applicable), including LIBOR floors (if applicable). Weighted average is based on outstanding principal as of December 31, 2020.

[4] Carrying value is presented net of loan loss reserves of $6.0 million.

[5] Includes a fixed rate loan with an outstanding principal balance of $27.2 million and a loan commitment of $39.7 million at September 30, 2021, which shares the same collateral as floating rate loans with an outstanding principal balance of $143.7 million and a loan commitment of $146.8 million at September 30, 2021.

[6] Senior loans include senior mortgages and similar credit quality loans, including related contiguous subordinate loans, and pari passu participations in senior mortgage loans.

[7] Includes a fixed rate loan with an outstanding principal balance of $6.4  million and a loan commitment of $39.7 million as of December 31, 2020, which shares the same collateral as floating rate loans with an outstanding principal balance of $138.3 million and a loan commitment of $146.8 million at December 31, 2020.

[8] One-month LIBOR as of September 30, 2021 was 0.08%. Weighted average is based on outstanding principal as of September 30, 2021.

[9] One-month LIBOR as of December 31, 2020 was 0.14%. Weighted average is based on outstanding principal as of December 31, 2020.