Investment in Loans by Credit Risk Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2015		Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	$ 1,524,159		$ 1,409,254
Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	1,433,092		1,328,134
Watchlist
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	91,067	[1]	81,120	[2]
Commercial mortgages
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	1,282,154	[3]	1,148,290	[4]
Commercial mortgages | Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	1,247,234		1,125,370
Commercial mortgages | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	34,920	[1]	22,920	[2]
Mezzanine Loans
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	202,694		226,105
Mezzanine Loans | Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	154,493		175,915
Mezzanine Loans | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	48,201	[1]	50,190	[2]
Preferred Equity Interests
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	39,311		34,859
Preferred Equity Interests | Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	31,365		26,849
Preferred Equity Interests | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance	$ 7,946	[1]	$ 8,010	[2]

[1] As of June 30, 2015, this includes $5,500 of unpaid principal relating to previously identified troubled debt restructurings (TDRs) that are on accrual status.

[2] As of December 31, 2014, this includes $5,500 of unpaid principal relating to previously identified TDRs that are on accrual status.

[3] Commercial mortgages includes eleven conduit loans with an unpaid principal balance and carrying amount of $114,048 a weighted-average coupon of 4.3% and maturity dates ranging from November 2022 through July 2025. These commercial mortgages are accounted for as loans held for sale.

[4] Commercial mortgages includes 11 conduit loans with an unpaid principal balance and carrying amount of $93,925, a weighted-average coupon of 4.6% and maturity dates ranging from November 2019 through January 2025. These commercial mortgages are accounted for as loans held for sale.