CRE Debt and Preferred Equity Investments - Based on Outstanding Principal (Detail) - USD ($) $ in Thousands		9 Months Ended			12 Months Ended
		Sep. 30, 2017		Sep. 30, 2016	Dec. 31, 2016
Real Estate [Line Items]
Net carrying value, beginning balance		$ 970,505	[1]	$ 1,348,817	$ 1,348,817
Originations & advances (principal)		327,717			273,708
Principal payments		(317,114)			(654,117)
Amortization & accretion of (premium) discounts		(15)			(314)
Net (increase) decrease in origination fees		(3,136)			(2,558)
Amortization of net origination fees		3,791		3,925	4,969
Net carrying value, ending balance		981,748			970,505	[1]
Preferred Equity Interests
Real Estate [Line Items]
Net carrying value, beginning balance		8,967	[1]	121,773	121,773
Principal payments					(113,444)
Amortization of net origination fees		13			638
Net carrying value, ending balance		8,980			8,967	[1]
Commercial Mortgage
Real Estate [Line Items]
Net carrying value, beginning balance	[1],[2]	970,505
Net carrying value, ending balance	[1],[2]	981,748			970,505
Commercial Mortgage | Mezzanine Loans
Real Estate [Line Items]
Net carrying value, beginning balance		451,467	[1],[2]	578,503	578,503
Originations & advances (principal)		63,801			62,390
Principal payments		(124,506)			(191,291)
Amortization & accretion of (premium) discounts		28			(178)
Net (increase) decrease in origination fees		(605)			(472)
Amortization of net origination fees		1,974			2,515
Net carrying value, ending balance	[2]	392,159			451,467	[1]
Commercial Mortgage | Senior Mortgages
Real Estate [Line Items]
Net carrying value, beginning balance		510,071	[1],[2]	385,838	385,838
Originations & advances (principal)		263,916			211,318
Principal payments		(192,608)			(86,310)
Amortization & accretion of (premium) discounts		(43)			(136)
Net (increase) decrease in origination fees		(2,531)			(2,086)
Amortization of net origination fees		1,804			1,447
Net carrying value, ending balance	[2]	580,609			510,071	[1]
Commercial Mortgage | Senior Securitized Mortgages
Real Estate [Line Items]
Net carrying value, beginning balance	[3]			$ 262,703	262,703
Principal payments	[3]				(263,072)
Amortization of net origination fees	[3]				369
Commercial Mortgage | Preferred Equity Interests
Real Estate [Line Items]
Net carrying value, beginning balance	[2]	8,967
Net carrying value, ending balance	[2]	$ 8,980			$ 8,967

[1] Excludes loans held for sale, net.

[2] Carrying value includes unamortized origination fees of $3.9 million and $4.5 million at September 30, 2017 and December 31, 2016, respectively.

[3] Assets of consolidated VIE.