Debt and Preferred Equity Investments - Rollforward of Net Book Balance (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance at beginning of period	$ 1,580,306	$ 2,099,393
Redemptions/Sales/Syndications/Equity Ownership/Amortization	(956,869)	(1,190,689)
Net change in loan loss reserves	(17,260)	4,000
Balance at end of period	1,153,363	1,580,306
Debt Investments in Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Originations/Accretion	383,365	652,866
Preferred equity investments
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Originations/Accretion	$ 163,821	$ 14,736