Commercial Mortgage, Subordinate Loans and Other Lending Assets, Net - Schedule of Mortgage Loans by Property Type and Geographic Distribution (Details) $ in Thousands, £ in Millions		12 Months Ended
		Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Nov. 30, 2020 USD ($)	Nov. 30, 2020 GBP (£)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 6,535,079	$ 6,375,093
% of Portfolio		100.00%	100.00%
General CECL allowance		$ (38,102)
Carrying value, net		6,496,977	$ 6,375,093
General CECL allowance related to unfunded commitments		3,365	0			$ 3,595	$ 4,119	$ 6,059	$ 3,088
Commercial Mortgage Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		5,468,098
General CECL allowance		(17,012)							(12,149)
Carrying value, net	[1],[2]	5,451,084	5,326,967
Subordinate Mortgage Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		1,066,981
General CECL allowance		(21,090)							$ (15,630)
Carrying value, net	[2]	1,045,893	1,048,126
New York City
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 2,370,337	$ 2,167,487
% of Portfolio		36.30%	34.00%
Northeast
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 103,567	$ 110,771
% of Portfolio		1.60%	1.70%
United Kingdom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 1,263,264	$ 1,274,390
% of Portfolio		19.30%	20.00%
West
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 749,985	$ 728,182
% of Portfolio		11.50%	11.40%
Midwest
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 552,537	$ 614,337
% of Portfolio		8.50%	9.60%
Southeast
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 581,301	$ 564,166
% of Portfolio		8.90%	8.90%
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 914,088	$ 915,760
% of Portfolio		13.90%	14.40%
Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 1,911,145	$ 1,401,400
% of Portfolio		29.20%	22.00%
Hotel
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 1,576,369	$ 1,660,162
% of Portfolio		24.10%	26.00%
Residential-For-Sale, Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 738,035	$ 692,816
% of Portfolio		11.30%	10.90%
Residential-for-sale: inventory
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 244,803	$ 321,673
% of Portfolio		3.70%	5.10%
Urban Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 655,456	$ 643,706
% of Portfolio		10.00%	10.10%
Carrying value, net				$ 422,700	£ 309.2
Healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 369,676	$ 371,423
% of Portfolio		5.70%	5.80%
Urban Predevelopment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 298,909	$ 409,864
% of Portfolio		4.60%	6.40%
Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 228,918	$ 227,940
% of Portfolio		3.50%	3.60%
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying Value		$ 511,768	$ 646,109
% of Portfolio		7.90%	10.10%

[1] Includes $5,418,999 and $4,852,087 pledged as collateral under secured debt arrangements in 2020 and 2019, respectively

[2] Net of $213,102 CECL Allowances in 2020, comprised of $175,000 Specific CECL Allowance and $38,102 General CECL Allowance. Net of $56,981 provision for loan loss in 2019.