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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Mortgage Loans On Real Estate [Line Items]
Beginning balance	$ 13,531	$ 18,165	$ 12,354	$ 17,097
Provision (benefit) for loan losses	20,863	1,344	22,398	2,669
Charge-offs, net of recoveries	(10,880)	(1,272)	(11,238)	(1,529)
Ending balance	23,514	18,237	23,514	18,237
Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	10,979	3,328	10,640	3,154
Provision (benefit) for loan losses	12,623	1,069	13,322	1,243
Charge-offs, net of recoveries	(10,880)		(11,240)
Ending balance	12,722	4,397	12,722	4,397
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	852	12,040		12,139
Provision (benefit) for loan losses	8,240	93	9,090	251
Charge-offs, net of recoveries		(1,272)	2	(1,529)
Ending balance	9,092	10,861	9,092	10,861
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Beginning balance	1,700	2,797	1,714	1,804
Provision (benefit) for loan losses		182	(14)	1,175
Ending balance	$ 1,700	$ 2,979	$ 1,700	$ 2,979