Investment in Loans by Credit Risk Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2016		Dec. 31, 2015
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 1,294,874
Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,162,233
Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	89,811
Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	42,830
Commercial Portfolio Segment
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,294,874		$ 1,627,121
Commercial Portfolio Segment | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,162,233	[1]	1,427,328	[2]
Commercial Portfolio Segment | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	89,811		169,556
Commercial Portfolio Segment | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	42,830		30,237
Commercial Portfolio Segment | Satisfactory
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,142,467		1,494,884
Commercial Portfolio Segment | Satisfactory | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,048,724		1,336,883
Commercial Portfolio Segment | Satisfactory | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	57,063		135,710
Commercial Portfolio Segment | Satisfactory | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	36,680		22,291
Commercial Portfolio Segment | Impaired
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	152,407		132,237
Commercial Portfolio Segment | Impaired | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	113,509		90,445
Commercial Portfolio Segment | Impaired | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	32,748		33,846
Commercial Portfolio Segment | Impaired | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 6,150		$ 7,946

[1] Commercial mortgage loans include two conduit loans with unpaid principal balances and carrying amounts totaling $20,181, a weighted-average coupon of 4.8%, and maturity dates ranging from June 2025 through December 2025. These commercial mortgage loans are accounted for as loans held for sale.

[2] Commercial mortgage loans include six conduit loans with unpaid principal balances and carrying amounts totaling $49,239, a weighted-average coupon of 4.8%, and maturity dates ranging from December 2020 through January 2026. These commercial mortgage loans are accounted for as loans held for sale.