Investment in Loans by Credit Risk Category (Detail) - Commercial Portfolio Segment - USD ($) $ in Thousands	Jun. 30, 2016		Dec. 31, 2015
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 1,496,821		$ 1,627,121
Commercial mortgage loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,315,301	[1]	1,427,328	[2]
Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	142,582		169,556
Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	38,938		30,237
Satisfactory
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,367,819		1,494,884
Satisfactory | Commercial mortgage loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,223,266		1,336,883
Satisfactory | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	113,529		135,710
Satisfactory | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	31,024		22,291
Watchlist
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	129,002		132,237
Watchlist | Commercial mortgage loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	92,035		90,445
Watchlist | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	29,053		33,846
Watchlist | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 7,914		$ 7,946

[1] Commercial mortgage loans includes five conduit loans with an unpaid principal balance and carrying amount of $41,455, a weighted-average coupon of 4.8% and maturity dates ranging from June 2025 through July 2026. These commercial mortgages are accounted for as loans held for sale.

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