Commercial Mortgage Loans, Subordinate Loans and Other Lending Assets, Net - Additional Information (Details) £ in Millions			1 Months Ended			3 Months Ended									12 Months Ended
		May 24, 2021 USD ($)	Jan. 31, 2020 USD ($)	Jan. 31, 2020 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 GBP (£)	Jun. 30, 2020 USD ($)	Jun. 30, 2020 GBP (£)	Mar. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2021 GBP (£)	Jan. 18, 2021 USD ($)	Dec. 31, 2020 GBP (£)	Nov. 30, 2020 USD ($)	Nov. 30, 2020 GBP (£)	Oct. 31, 2020 USD ($)	Dec. 31, 2015 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
Mortgage loans on real estate, commercial and consumer, percentage of portfolio															100.00%	100.00%
Principal balance						$ 7,890,848,000		$ 6,535,079,000							$ 7,890,848,000	$ 6,535,079,000
Reversal of Specific CECL Allowance		$ 10,000,000													(30,000,000)	118,019,000	$ 13,000,000
Total carrying value, net						7,857,260,000		6,496,977,000							7,857,260,000	6,496,977,000
Amortized cost of loans in cost recovery						404,800,000		373,500,000							404,800,000	373,500,000
Interest Receivable						41,219,000		40,559,000							41,219,000	40,559,000
Interest received for loans in cost recovery															1,400,000	1,800,000
Payment in kind interest															47,700,000	46,700,000	54,600,000
General CECL (allowance) reversal															4,773,000	10,600,000
Proceeds from pre-payment penalties or accelerated fees															1,500,000	200,000	6,100,000
Proceed from shared appreciation															3,700,000
Realized loss on investment															20,767,000	42,465,000
Equity method investment, (reversal)																(3,019,000)
Sale of Mezzanine Loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Total carrying value, net						41,900,000									41,900,000
Realized loss on investment											$ 0				(800,000)
Proceeds from sale of loan			$ 81,300,000	£ 62.2	$ 30,300,000						81,300,000	£ 62.2
Sale of Unfunded Commitment of Senior Mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Proceeds from sale of loan			$ 65,300,000	£ 50.0							65,300,000	£ 50.0
Sale Of Mezzanine Loan Two
Financing Receivable, Allowance for Credit Loss [Line Items]
Realized loss on investment											$ 0
Proceeds from sale of loan					122,300,000
Sale of Construction Loan Interests
Financing Receivable, Allowance for Credit Loss [Line Items]
Commitments sold									$ 376,900,000
Funded amount of interests sold									127,000,000
Gain (loss) on loan interests sold									1,400,000
Sale of Inventory Loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Gain (loss) on loan interests sold									1,000,000
Outstanding principal on loan									$ 124,200,000	£ 97.5
Sale Of Residential-For-Sale Inventory Loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Total carrying value, net								73,400,000								73,400,000
Realized loss on investment								2,700,000
Commercial Mortgage and Subordinated Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Principal balance						7,890,848,000		6,535,079,000							7,890,848,000	6,535,079,000
Reversal of Specific CECL Allowance															30,000,000
Total carrying value, net						97,800,000		136,100,000							97,800,000	136,100,000
Realized loss on investment															(20,767,000)
Subordinate Mortgage Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Principal balance						855,982,000									855,982,000
Total carrying value, net	[1]					844,948,000		1,045,893,000							844,948,000	1,045,893,000
Payment in kind interest															0	800,000
Maximum exposure to loss						64,600,000		68,100,000							$ 64,600,000	$ 68,100,000
Maximum exposure to loss, term															4 years 6 months	5 years 9 months 18 days
Subordinate Mortgage Portfolio Segment | Residential Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Debt related to real estate owned, held for sale						31,200,000									$ 31,200,000
Realized loss on investment						0
Urban Retail
Financing Receivable, Allowance for Credit Loss [Line Items]
Mortgage loans on real estate, commercial and consumer, percentage of portfolio															9.00%	10.00%
Principal balance						711,592,000		655,456,000							$ 711,592,000	$ 655,456,000
Total carrying value, net																					$ 422,700,000	£ 309.2
Urban Retail | Commercial Mortgage and Subordinated Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Total carrying value, net						352,800,000		338,400,000							352,800,000	338,400,000		£ 260.7		£ 247.5	$ 338,400,000	£ 247.5
Past Due 30-59
Financing Receivable, Allowance for Credit Loss [Line Items]
Amortized cost of loans in cost recovery								19,000,000								19,000,000
Past Due 90
Financing Receivable, Allowance for Credit Loss [Line Items]
Amortized cost of loans in cost recovery						757,600,000		711,900,000							757,600,000	711,900,000			$ 352,800,000
Hotel - New York City
Financing Receivable, Allowance for Credit Loss [Line Items]
Principal balance																								$ 157,800,000
Financing receivable, contribution of additional equity, borrowing of loan															15,000,000
Financing Receivable, Troubled Debt Restructuring						142,800,000		142,800,000							142,800,000	142,800,000
Amortized cost prior to Specific CECL Allowance						145,300,000		144,700,000							145,300,000	144,700,000
Reversal of Specific CECL Allowance							$ (15,000,000)
Hotel - Pittsburgh, PA
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, contribution of additional equity, borrowing of loan														$ 11,000,000
Financing Receivable, Troubled Debt Restructuring						25,700,000		21,600,000							25,700,000	21,600,000
Amortized cost prior to Specific CECL Allowance						25,600,000		21,500,000							25,600,000	21,500,000
Reversal of Specific CECL Allowance								$ (9,500,000)						9,500,000
Total carrying value, net														38,500,000
Payments for (proceeds from) loans receivable														$ 5,900,000
Commitment to lend						11,400,000									11,400,000
Retail New York | Sale of Mezzanine Loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Total carrying value, net					$ 470,800,000												470,800,000
Proceeds from sale of loan																	30,300,000
Transferor's continuing involvement, arrangements of financial support, amount						318,100,000									318,100,000
Retail New York | Sale Of Mezzanine Loan Two
Financing Receivable, Allowance for Credit Loss [Line Items]
Proceeds from sale of loan																	$ 122,300,000
Residential Condominium - Bethesda, MD
Financing Receivable, Allowance for Credit Loss [Line Items]
Principal balance																							$ 80,000,000
Reversal of Specific CECL Allowance																1,100,000
Gain (loss) on loan interests sold															$ (14,100,000)
Residential Condominium - Bethesda, MD | Commercial Mortgage and Subordinated Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Reversal of Specific CECL Allowance																11,100,000
Realized loss on investment																$ 11,100,000
Floating Rate Loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Mortgage loans on real estate, commercial and consumer, percentage of portfolio															98.00%	95.00%
Mezzanine Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total carrying value, net						473,200,000									$ 473,200,000
Debt instrument, face amount						471,000,000									471,000,000
Payment in kind interest															178,000,000
Mezzanine Loans | Subordinate Mortgage Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Reversal of Specific CECL Allowance													$ 10,000,000
Senior Mezzanine Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total carrying value, net						238,400,000									238,400,000
Junior Mezzanine A Loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Total carrying value, net						152,800,000									152,800,000
Junior Mezzanine B Loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Total carrying value, net						$ 82,000,000									$ 82,000,000

[1] Net of $178,588 and $213,102 CECL Allowances in 2021 and 2020, respectively, comprised of $145,000 and $175,000 Specific CECL Allowance and $33,588 and $38,102 General CECL Allowance, respectively.