Commercial Real Estate Loans - Concentration of Credit Risk (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) loan	Dec. 31, 2020 USD ($) loan
Loans held-for-investment
Concentration Risk [Line Items]
Loan Count (in loans) | loan	52	42
Outstanding Principal | $	$ 5,478,044	$ 4,869,696
Loans held-for-investment | Mezzanine loans
Concentration Risk [Line Items]
Loan Count (in loans) | loan	5	4
Outstanding Principal | $	$ 107,217	$ 90,329
Multifamily | Loans held-for-investment | Mezzanine loans
Concentration Risk [Line Items]
Loan Count (in loans) | loan	1
Outstanding Principal | $	$ 31,600	$ 50,000
Loans held-for-investment | Geographic concentration risk
Concentration Risk [Line Items]
Concentration of credit risk	100.00%	100.00%
Loans held-for-investment | Geographic concentration risk | Texas
Concentration Risk [Line Items]
Concentration of credit risk	14.00%	6.60%
Loans held-for-investment | Geographic concentration risk | New York
Concentration Risk [Line Items]
Concentration of credit risk	13.60%	14.50%
Loans held-for-investment | Geographic concentration risk | California
Concentration Risk [Line Items]
Concentration of credit risk	12.40%	7.70%
Loans held-for-investment | Geographic concentration risk | Massachusetts
Concentration Risk [Line Items]
Concentration of credit risk	9.50%	8.40%
Loans held-for-investment | Geographic concentration risk | Pennsylvania
Concentration Risk [Line Items]
Concentration of credit risk	9.50%	9.70%
Loans held-for-investment | Geographic concentration risk | Virginia
Concentration Risk [Line Items]
Concentration of credit risk	7.80%	10.10%
Loans held-for-investment | Geographic concentration risk | Colorado
Concentration Risk [Line Items]
Concentration of credit risk	7.30%	4.70%
Loans held-for-investment | Geographic concentration risk | Florida
Concentration Risk [Line Items]
Concentration of credit risk	4.80%	5.70%
Loans held-for-investment | Geographic concentration risk | Illinois
Concentration Risk [Line Items]
Concentration of credit risk	4.30%	11.80%
Loans held-for-investment | Geographic concentration risk | Washington
Concentration Risk [Line Items]
Concentration of credit risk	4.00%	7.20%
Loans held-for-investment | Geographic concentration risk | Minnesota
Concentration Risk [Line Items]
Concentration of credit risk	3.60%	4.00%
Loans held-for-investment | Geographic concentration risk | Washington D.C.
Concentration Risk [Line Items]
Concentration of credit risk	3.20%	2.90%
Loans held-for-investment | Geographic concentration risk | Oregon
Concentration Risk [Line Items]
Concentration of credit risk	2.00%	2.30%
Loans held-for-investment | Geographic concentration risk | Georgia
Concentration Risk [Line Items]
Concentration of credit risk	1.60%	1.80%
Loans held-for-investment | Geographic concentration risk | Alabama
Concentration Risk [Line Items]
Concentration of credit risk	1.20%	1.40%
Loans held-for-investment | Geographic concentration risk | Arizona
Concentration Risk [Line Items]
Concentration of credit risk	1.00%	0.00%
Loans held-for-investment | Geographic concentration risk | Other U.S.
Concentration Risk [Line Items]
Concentration of credit risk	0.20%	1.20%
Loans held-for-investment | Product concentration risk
Concentration Risk [Line Items]
Concentration of credit risk	100.00%	100.00%
Loans held-for-investment | Product concentration risk | Multifamily
Concentration Risk [Line Items]
Concentration of credit risk	45.00%	51.00%
Loans held-for-investment | Product concentration risk | Office
Concentration Risk [Line Items]
Concentration of credit risk	24.40%	30.20%
Loans held-for-investment | Product concentration risk | Life Science
Concentration Risk [Line Items]
Concentration of credit risk	7.80%	0.00%
Loans held-for-investment | Product concentration risk | Hospitality
Concentration Risk [Line Items]
Concentration of credit risk	6.80%	4.50%
Loans held-for-investment | Product concentration risk | Industrial
Concentration Risk [Line Items]
Concentration of credit risk	5.50%	1.80%
Loans held-for-investment | Product concentration risk | Condo (Residential)
Concentration Risk [Line Items]
Concentration of credit risk	4.60%	6.10%
Loans held-for-investment | Product concentration risk | Student Housing
Concentration Risk [Line Items]
Concentration of credit risk	3.60%	1.40%
Loans held-for-investment | Product concentration risk | Retail
Concentration Risk [Line Items]
Concentration of credit risk	2.10%	5.00%
Loans held-for-investment | Product concentration risk | Single Family Rental
Concentration Risk [Line Items]
Concentration of credit risk	0.20%	0.00%
Loans held-for-investment, excluded | Product concentration risk | Multifamily
Concentration Risk [Line Items]
Concentration of credit risk	0.60%	1.00%