Roll-Forwards of Allowance for Losses for Commercial Mortgages Mezzanine Loans, and Other Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Mortgage Loans On Real Estate [Line Items]
Beginning balance	$ 12,796	$ 15,336	$ 9,218	$ 22,955
Provision for loan losses	1,850	1,500	5,850	3,500
Charge-offs, net of recoveries		(1,174)	(422)	(10,793)
Ending balance	14,646	15,662	14,646	15,662
Commercial mortgages
Mortgage Loans On Real Estate [Line Items]
Provision for loan losses	3,154		3,154
Ending balance	3,154		3,154
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	11,470	14,017	7,892	21,636
Provision for loan losses	(1,255)	1,132	2,745	3,132
Charge-offs, net of recoveries		(813)	(422)	(10,432)
Ending balance	10,215	14,336	10,215	14,336
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Beginning balance	1,326	1,319	1,326	1,319
Provision for loan losses	(49)	368	(49)	368
Charge-offs, net of recoveries		(361)		(361)
Ending balance	$ 1,277	$ 1,326	$ 1,277	$ 1,326