LOANS - CRE Debt and Preferred Equity Investments - Based on Outstanding Principal (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2019	Dec. 31, 2018
Real Estate [Line Items]
Beginning balance	[1],[2]	$ 4,585,975
Principal payments		(456,300)
Ending balance	[2]	3,946,614	$ 4,585,975	[1]
Commercial Mortgage
Real Estate [Line Items]
Beginning balance		1,296,803	1,029,327
Originations & advances (principal)		1,362,156	628,177
Principal payments		(306,104)	(353,424)
Transfers		(922,524)
Net (increase) decrease in origination fees		(4,211)	(6,994)
Amortization of net origination fees		4,521	3,213
Allowance for loan losses		(9,207)	(3,496)
Ending balance		1,421,434	1,296,803
Commercial Mortgage | Senior mortgages
Real Estate [Line Items]
Beginning balance		981,202	625,900
Originations & advances (principal)		376,651	575,953
Principal payments		(16,099)	(216,849)
Transfers		(913,849)
Net (increase) decrease in origination fees		2,972	(6,624)
Amortization of net origination fees		1,583	2,822
Allowance for loan losses		0	0
Ending balance		432,460	981,202
Commercial Mortgage | Senior Securitized Mortgages
Real Estate [Line Items]
Beginning balance		0
Originations & advances (principal)		964,473
Principal payments		(150,050)
Transfers		0
Net (increase) decrease in origination fees		(6,999)
Amortization of net origination fees		2,581
Allowance for loan losses		0
Ending balance		810,005	0
Commercial Mortgage | Preferred equity
Real Estate [Line Items]
Beginning balance		0	8,985
Originations & advances (principal)			0
Principal payments			(9,000)
Net (increase) decrease in origination fees			0
Amortization of net origination fees			15
Allowance for loan losses			0
Ending balance			0
Commercial Mortgage | Mezzanine loans
Real Estate [Line Items]
Beginning balance		315,601	394,442
Originations & advances (principal)		21,032	52,224
Principal payments		(139,955)	(127,575)
Transfers		(8,675)
Net (increase) decrease in origination fees		(184)	(370)
Amortization of net origination fees		357	376
Allowance for loan losses		(9,207)	(3,496)
Ending balance		$ 178,969	$ 315,601

[1] ⁽¹⁾  Derived from the audited consolidated financial statements at December 31, 2018.

[2] ⁽⁴⁾  Includes $76.3 million and $97.5 million of residential mortgage loans held for sale and $0 and $42.2 million of commercial mortgage loans held for sale at September 30, 2019 and December 31, 2018, respectively.