Roll-Forwards of Allowance for Loan Losses for Commercial Mortgage Loans, Mezzanine Loans and Preferred Equity Interests (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Mortgage Loans On Real Estate [Line Items]
Beginning balance	$ 23,514	$ 18,237	$ 12,354	$ 17,097
Provision (benefit) for loan losses	5,516	1,533	27,914	4,202
Charge-offs, net of recoveries	(3,890)	(1,115)	(15,128)	(2,644)
Ending balance	25,140	18,655	25,140	18,655
Commercial mortgage loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	12,722	4,397	10,640	3,154
Provision (benefit) for loan losses	3,368	1,957	16,690	3,200
Charge-offs, net of recoveries	0	(10)	(11,240)	(10)
Ending balance	16,090	6,344	16,090	6,344
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	9,092	10,861	0	12,139
Provision (benefit) for loan losses	2,148	(394)	11,238	(143)
Charge-offs, net of recoveries	(3,890)	(1,105)	(3,888)	(2,634)
Ending balance	7,350	9,362	7,350	9,362
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Beginning balance	1,700	2,979	1,714	1,804
Provision (benefit) for loan losses	0	(30)	(14)	1,145
Charge-offs, net of recoveries	0	0	0	0
Ending balance	$ 1,700	$ 2,949	$ 1,700	$ 2,949