Secured Debt Agreements - Summary of Statistics for Asset-Specific Repurchase Agreements (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) Loans	Dec. 31, 2014 USD ($) Loans
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Number of loans | Loans	3	4
Principal balance	$ 308,786	$ 429,197
Weighted-average cash coupon	L+5.01 %
Weighted-average cost / all-in yield	L+5.48 %
Collateral Assets [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon, Basis spread on variable rate	5.01%	5.07%
Weighted-average cost / all-in yield, rate	5.48%	5.53%
Asset-Specific Repurchase Agreements [Member]
Participating Mortgage Loans [Line Items]
Number of loans | Loans	3	3
Principal balance	$ 235,318	$ 324,553
Weighted-average cash coupon	L+2.64 %
Weighted-average cost / all-in yield	L+3.06 %
Asset-Specific Repurchase Agreements [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon, Basis spread on variable rate	2.64%	2.68%
Weighted-average cost / all-in yield, rate	3.06%	3.16%