Loans and Preferred Equity Held for Investment, net - Narrative (Details)	3 Months Ended				12 Months Ended
	Dec. 31, 2020 USD ($) loan co-investment	Sep. 30, 2020 USD ($) loan	Jun. 30, 2020 USD ($) co-investment	Mar. 31, 2020 USD ($) property loan	Dec. 31, 2020 USD ($) investment loan co-investment	Dec. 31, 2019 USD ($) loan property	Dec. 31, 2018 USD ($)	Mar. 31, 2018 loan	Dec. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maturity period of debt instruments depending upon the asset type (in years)					3 years 4 months 24 days
Interest receivable	$ 7,000,000.0				$ 7,000,000.0	$ 9,800,000
Provision for loan losses					78,561,000	220,572,000	$ 113,911,000
Allowance for loan losses						272,600,000
Effect of CECL adoption						21,093,000	0		$ 0
Carrying value of loans						409,700,000
Assets-held-for-sale, including discontinued operations	$ 0				$ 0	$ 0
Real estate debt investments with contractual payments past due | loan	0				0	5
Number of CRE debt investments contributed to more than 10% of interest income | investment					0
Percent of interest income contributed by investment					10.00%
Future funding commitments	$ 163,000,000.0				$ 163,000,000.0	$ 276,600,000
Allowance for lending commitments	1,300,000				1,300,000
Core
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses			$ 1,500,000	$ 1,500,000
Allowance for loan losses	1,600,000				1,600,000
Effect of CECL adoption	100,000				100,000
Core | Corporate term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from collection on loans	12,100,000
Number of loans classified as held-for-sale | loan		1
COVID-19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses				36,800,000
NY hospitality loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses						$ 154,300,000	53,800,000
Midwest hospitality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Bridge loan		$ 19,500,000
Midwest hospitality | Core
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sale of receivables		24,500,000
Northeast Office Portfolio | Core
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses				12,100,000
Allowance for loan losses				20,900,000
Effect of CECL adoption				$ 8,800,000
Number of office buildings				6
Proceeds from collection on loans		80,700,000
Number of loans classified as held-for-sale				1
West Hospitality | Core
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses	38,000,000.0
Proceeds from sale of receivables	105,200,000			$ 2,600,000
Gain (loss) on sales of loans, net	1,500,000		32,800,000
Allowance for loan losses			$ 5,200,000
Effect of CECL adoption				$ 2,600,000
Number of loans classified as held-for-sale | co-investment			1
Legacy, non-strategic portfolio | Midwest Regional Mall | Legacy, Non-Strategic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans, non-accrual status | loan						1
Impaired financing receivable, recorded impairment						$ 10,600,000
Proceeds from sale of receivables			$ 8,300,000
Gain (loss) on sales of loans, net			$ 3,700,000
NY hospitality loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans, non-accrual status | loan						4
NY hospitality loans | Legacy, non-strategic portfolio
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans, non-accrual status | loan				4				4
Commercial mortgage, regional mall loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses						$ 26,700,000
Number of loans with potential loss | loan						3
Number of properties collateralized | property						2
Commercial mortgage, regional mall loans | Northeast Regional Mall B
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses						$ 10,500,000	$ 8,800,000
Proceeds from sale of receivables				$ 9,200,000
Gain (loss) on sales of loans, net				1,800,000
Commercial mortgage, regional mall loans | West Regional Mall
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses				$ 18,500,000
Proceeds from sale of receivables		23,500,000
Gain (loss) on sales of loans, net		$ 6,500,000
Number of loans with potential loss | loan						2
Number of properties collateralized | property				1
Commercial mortgage, regional mall loans | South Regional Mall A
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sale of receivables				$ 22,000,000.0
Gain (loss) on sales of loans, net				(1,600,000)
Commercial mortgage, regional mall loans | South Regional Mall B
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sale of receivables				13,500,000
Gain (loss) on sales of loans, net				$ 8,700,000
Commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value of loans						$ 377,421,000
Future funding commitments	$ 163,000,000.0				$ 163,000,000.0
Hospitality | Midwest hospitality | Core
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans, non-accrual status | co-investment	1				1
Provision for loan losses					$ 2,300,000