Loans and Preferred Equity Held for Investment, net - Narrative (Details)	1 Months Ended	3 Months Ended				9 Months Ended		12 Months Ended
	Nov. 06, 2020 USD ($)	Sep. 30, 2020 USD ($) loan	Jun. 30, 2020 USD ($) loan	Mar. 31, 2020 USD ($) loan building	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) loan investment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) loan property	Dec. 31, 2018 USD ($)	Mar. 31, 2018 loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maturity period of debt instruments depending upon the asset type (in years)						3 years 7 months 6 days
Interest receivable		$ 7,600,000				$ 7,600,000		$ 9,800,000
Provision for loan losses		10,404,000			$ 110,314,000	80,285,000	$ 220,572,000
Allowance for loan losses								272,600,000
Effect of CECL adoption						21,093,000	$ 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		$ 173,200,000				$ 173,200,000		$ 276,600,000
Allowance for lending commitments		$ 1,200,000				1,200,000
Average
Accounts, Notes, Loans and Financing Receivable [Line Items]
Internal credit risk rating		3.7
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
Core | Corporate term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans classified as held-for-sale | loan			1
Core | Corporate term loans | Subsequent event
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from collection on loans	$ 12,100,000
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				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
Number of loans with potential loss | loan				1
Number of properties collateralized | building				6
Allowance for loan losses				$ 20,900,000
Effect of CECL adoption				8,800,000
Proceeds from collection on loans		80,700,000
West Hospitality | Core
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses		38,000,000.0		2,600,000
Proceeds from sale of receivables		105,200,000
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 | loan			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		$ 173,200,000				$ 173,200,000
Hospitality | Midwest hospitality | Core
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans, non-accrual status | loan								1
Provision for loan losses				$ 2,300,000