CRE Debt and Preferred Equity Investments -Based on Outstanding Principal (Detail) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Transaction to Real Estate Investments [Line Items]
Beginning balance		$ 1,348,817	$ 1,518,165
Originations & advances (principal)		180,962	489,237
Principal payments		(351,930)	(625,642)
Sales (principal)			(46,945)
Amortization & accretion of (premium) discounts		(255)	143
Net (increase) decrease in origination fees		(1,863)	(8,787)
Amortization of net origination fees		1,737	4,146
Transfers			18,500
Allowance for loan losses		0	0
Net carrying value	[1]	1,177,468	1,348,817
Preferred Equity Interests
Transaction to Real Estate Investments [Line Items]
Beginning balance		121,773	212,905
Principal payments		(113,445)	(92,210)
Amortization & accretion of (premium) discounts			517
Amortization of net origination fees		625	561
Allowance for loan losses		0	0
Net carrying value	[1]	8,953	121,773
Commercial Mortgage
Transaction to Real Estate Investments [Line Items]
Net carrying value	[1],[2]	1,177,468	1,348,817
Commercial Mortgage | Mezzanine Loans
Transaction to Real Estate Investments [Line Items]
Beginning balance		578,503	522,731
Originations & advances (principal)		25,897	195,312
Principal payments		(122,408)	(153,693)
Amortization & accretion of (premium) discounts		(221)	(232)
Net (increase) decrease in origination fees		(285)	(4,806)
Amortization of net origination fees		615	691
Transfers			18,500
Allowance for loan losses		0	0
Net carrying value	[1],[2]	482,101	578,503
Commercial Mortgage | Senior Mortgages
Transaction to Real Estate Investments [Line Items]
Beginning balance		385,838	383,895
Originations & advances (principal)		155,065	293,925
Principal payments		(65,077)	(243,270)
Sales (principal)			(46,945)
Amortization & accretion of (premium) discounts		(34)	(142)
Net (increase) decrease in origination fees		(1,578)	(3,702)
Amortization of net origination fees		345	2,077
Allowance for loan losses		0	0
Net carrying value	[1],[2]	474,559	385,838
Commercial Mortgage | Senior Securitized Mortgages
Transaction to Real Estate Investments [Line Items]
Beginning balance	[3]	262,703	398,634
Principal payments	[3]	(51,000)	(136,469)
Net (increase) decrease in origination fees	[3]		(279)
Amortization of net origination fees	[3]	152	817
Allowance for loan losses	[3]	0	0
Net carrying value	[1],[2],[3],[4]	211,855	262,703
Commercial Mortgage | Preferred Equity Interests
Transaction to Real Estate Investments [Line Items]
Net carrying value	[2]	$ 8,953	$ 121,773

[1] Excludes Loans held for sale.

[2] Carrying value includes unamortized origination fees of $7.1 million and $8.1 million as of March 31, 2016 and December 31, 2015, respectively.

[3] Assets of consolidated VIE.

[4] Assets of consolidated VIEs.