CRE Debt and Preferred Equity Investments -Based on Outstanding Principal (Detail) - USD ($) $ in Thousands			9 Months Ended		12 Months Ended
		Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Transaction to Real Estate Investments [Line Items]
Beginning balance		$ 1,348,817	$ 1,348,817		$ 1,518,165
Originations & advances (principal)			206,031		489,237
Principal payments		(50,500)	(486,434)		(625,642)
Sales (principal)					(46,945)
Amortization & accretion of (premium) discounts			(294)		143
Net (increase) decrease in origination fees			(1,848)		(8,787)
Amortization of net origination fees			3,925	$ 3,350	4,146
Transfers					18,500
Net carrying value	[1]	1,070,197	1,070,197		1,348,817
Preferred Equity Interests
Transaction to Real Estate Investments [Line Items]
Beginning balance		121,773	121,773		212,905
Principal payments			(113,444)		(92,210)
Amortization & accretion of (premium) discounts					517
Amortization of net origination fees			633		561
Net carrying value	[1]	8,962	8,962		121,773
Commercial Mortgage
Transaction to Real Estate Investments [Line Items]
Net carrying value	[1],[2]	1,070,197	1,070,197		1,348,817
Commercial Mortgage | Mezzanine Loans
Transaction to Real Estate Investments [Line Items]
Beginning balance		578,503	578,503		522,731
Originations & advances (principal)			46,858		195,312
Principal payments			(160,585)		(153,693)
Amortization & accretion of (premium) discounts			(192)		(232)
Net (increase) decrease in origination fees			(282)		(4,806)
Amortization of net origination fees			1,868		691
Transfers					18,500
Net carrying value	[1],[2]	466,170	466,170		578,503
Commercial Mortgage | Senior Mortgages
Transaction to Real Estate Investments [Line Items]
Beginning balance		385,838	385,838		383,895
Originations & advances (principal)			159,173		293,925
Principal payments			(78,229)		(243,270)
Sales (principal)					(46,945)
Amortization & accretion of (premium) discounts			(102)		(142)
Net (increase) decrease in origination fees			(1,566)		(3,702)
Amortization of net origination fees			1,055		2,077
Net carrying value	[1],[2]	466,169	466,169		385,838
Commercial Mortgage | Senior Securitized Mortgages
Transaction to Real Estate Investments [Line Items]
Beginning balance	[3]	262,703	262,703		398,634
Principal payments	[3]		(134,176)		(136,469)
Net (increase) decrease in origination fees	[3]				(279)
Amortization of net origination fees	[3]		369		817
Net carrying value	[1],[2],[3],[4]	128,896	128,896		262,703
Commercial Mortgage | Preferred Equity Interests
Transaction to Real Estate Investments [Line Items]
Net carrying value	[2]	$ 8,962	$ 8,962		$ 121,773

[1] Excludes Loans held for sale, net.

[2] Carrying value includes unamortized origination fees of $4.9 million and $6.9 million as of September 30, 2016 and December 31, 2015, respectively.

[3] Assets of consolidated VIE.

[4] Assets of consolidated VIEs.