CRE Debt and Preferred Equity Investments (Detail) (USD $)	Dec. 31, 2014		Dec. 31, 2013
Transaction to Real Estate Investments [Line Items]
Carrying Value	$ 1,518,165,000		$ 1,583,969,000
Preferred Equity Interests
Transaction to Real Estate Investments [Line Items]
Carrying Value			247,160,000
Commercial Mortgage
Transaction to Real Estate Investments [Line Items]
Outstanding Principal	1,520,972,000		1,587,223,000
Carrying Value	1,518,165,000	[1]	1,583,969,000	[1]
Percentage of Loan Portfolio	100.00%	[2]	100.00%	[2]
Commercial Mortgage | Senior Mortgages
Transaction to Real Estate Investments [Line Items]
Outstanding Principal	384,304,000		669,512,000
Carrying Value	383,895,000	[1]	667,299,000	[1]
Percentage of Loan Portfolio	25.20%	[2]	42.20%	[2]
Commercial Mortgage | Senior Securitized Mortgages
Transaction to Real Estate Investments [Line Items]
Outstanding Principal	399,541,000	[3]
Carrying Value	398,634,000	[1],[3]
Percentage of Loan Portfolio	26.30%	[2],[3]	0.00%	[2],[3]
Commercial Mortgage | Subordinated Notes
Transaction to Real Estate Investments [Line Items]
Outstanding Principal			41,059,000
Carrying Value			41,408,000	[1]
Percentage of Loan Portfolio	0.00%	[2]	2.60%	[2]
Commercial Mortgage | Mezzanine Loans
Transaction to Real Estate Investments [Line Items]
Outstanding Principal	522,474,000		626,883,000
Carrying Value	522,731,000	[1]	628,102,000	[1]
Percentage of Loan Portfolio	34.40%	[2]	39.50%	[2]
Commercial Mortgage | Preferred Equity Interests
Transaction to Real Estate Investments [Line Items]
Outstanding Principal	214,653,000		249,769,000
Carrying Value	$ 212,905,000	[1]	$ 247,160,000	[1]
Percentage of Loan Portfolio	14.10%	[2]	15.70%	[2]

[1] Carrying value includes unamortized origination fees of $3.0 million and $4.9 million as of December 31, 2014 and December 31, 2013, respectively.

[2] Based on outstanding principal.

[3] Assets of consolidated VIE.