Loans and Preferred Equity Held for Investment, net - Summary of Loans Held for Investment, Net (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	$ 2,149,018	$ 2,858,423
Loan carrying value	2,143,938	2,848,956
Allowance for loan losses	(40,524)	(272,624)	$ (283,208)	$ (109,328)
Loans and preferred equity held for investment, net	2,103,414	2,576,332
Fixed rate
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	182,377	351,351
Variable rate
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	1,966,641	2,507,072
Commercial mortgage
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	2,149,018	2,858,423
Loans and preferred equity held for investment, net	2,143,938	2,848,956
Mezzanine loans | Fixed rate
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	$ 150,335	$ 223,395
Weighted average coupon (in percentage)	12.80%	12.80%
Weighted average maturity in years	4 years 2 months 12 days	4 years 2 months 12 days
Mezzanine loans | Variable rate
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	$ 18,178	$ 38,110
Weighted average coupon (in percentage)	9.40%	11.40%
Weighted average maturity in years	2 years 2 months 12 days	2 years
Preferred equity interests | Fixed rate
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	$ 18,350	$ 115,384
Weighted average coupon (in percentage)	15.00%	12.50%
Weighted average maturity in years	2 years 10 months 24 days	6 years 10 months 24 days
Preferred equity interests | Variable rate
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	$ 1,569
Weighted average coupon (in percentage)	5.30%
Weighted average maturity in years	3 months 18 days
Other loans | Fixed rate
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	$ 13,692	$ 12,572
Weighted average coupon (in percentage)	15.00%	15.00%
Weighted average maturity in years	3 years 8 months 12 days	4 years 4 months 24 days
Senior loans | Variable rate
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	$ 974,207	$ 1,462,467
Weighted average coupon (in percentage)	5.50%	6.00%
Weighted average maturity in years	3 years 4 months 24 days	3 years 9 months 18 days
Securitized loans | Variable rate
Financing Receivable, Impaired [Line Items]
Principal amount, financial assets	$ 972,687	$ 1,006,495
Weighted average coupon (in percentage)	5.10%	5.20%
Weighted average maturity in years	3 years 7 months 6 days	4 years 2 months 12 days
Loans and preferred equity held for investment | Fixed rate
Financing Receivable, Impaired [Line Items]
Loan carrying value	$ 181,635	$ 350,264
Loans and preferred equity held for investment | Variable rate
Financing Receivable, Impaired [Line Items]
Loan carrying value	1,962,303	2,498,692
Loans and preferred equity held for investment | Commercial mortgage
Financing Receivable, Impaired [Line Items]
Loan carrying value	2,143,938	2,848,956
Loans and preferred equity held for investment | Mezzanine loans | Fixed rate
Financing Receivable, Impaired [Line Items]
Loan carrying value	149,678	222,503
Loans and preferred equity held for investment | Mezzanine loans | Variable rate
Financing Receivable, Impaired [Line Items]
Loan carrying value	18,298	38,258
Loans and preferred equity held for investment | Preferred equity interests | Fixed rate
Financing Receivable, Impaired [Line Items]
Loan carrying value	18,350	115,313
Loans and preferred equity held for investment | Preferred equity interests | Variable rate
Financing Receivable, Impaired [Line Items]
Loan carrying value	1,569
Loans and preferred equity held for investment | Other loans | Fixed rate
Financing Receivable, Impaired [Line Items]
Loan carrying value	13,607	12,448
Loans and preferred equity held for investment | Senior loans | Variable rate
Financing Receivable, Impaired [Line Items]
Loan carrying value	971,963	1,457,738
Loans and preferred equity held for investment | Securitized loans | Variable rate
Financing Receivable, Impaired [Line Items]
Loan carrying value	$ 970,473	$ 1,002,696