Roll-Forwards of Allowance for Loan Losses for Commercial Mortgage Loans, Mezzanine Loans and Preferred Equity Interests (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Mortgage Loans On Real Estate [Line Items]
Beginning balance	$ 18,165	$ 10,797	$ 17,097	$ 9,218
Provision for loan losses	1,344	2,000	2,669	4,000
Charge-offs, net of recoveries	(1,272)	(1)	(1,529)	(422)
Ending balance	18,237	12,796	18,237	12,796
Commercial mortgage loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	3,328		3,154
Provision for loan losses	1,069		1,243
Ending balance	4,397		4,397
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	12,040	9,471	12,139	7,892
Provision for loan losses	93	2,000	251	4,000
Charge-offs, net of recoveries	(1,272)	(1)	(1,529)	(422)
Ending balance	10,861	11,470	10,861	11,470
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Beginning balance	2,797	1,326	1,804	1,326
Provision for loan losses	182		1,175
Ending balance	$ 2,979	$ 1,326	$ 2,979	$ 1,326