Loan Participations Sold - Summary of Statistics for Loan Participations Sold (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) Loans	Dec. 31, 2014 USD ($) Loans
Participating Mortgage Loans [Line Items]
Participations Sold, Number of loans | Loans	3	4
Principal balance, Participations Sold	$ 507,200	$ 499,433
Weighted-average cash coupon, Participations Sold	L+2.38 %
Weighted-average all-in cost / yield, Participations Sold	L+2.49 %
Underlying Loans, Number of loans | Loans	3	4
Principal balance, Underlying Loans	$ 618,470	$ 635,701
Weighted-average cash coupon, Underlying Loans	L+3.93 %
Weighted-average all-in cost / yield, Underlying Loans	L+4.15 %
LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon, Participations Sold, rate	2.38%	2.51%
Weighted-average all-in cost / yield, Participations Sold	2.49%	2.71%
Weighted-average cash coupon, Underlying Loans, rate	3.93%	4.10%
Weighted-average all-in cost / yield, Underlying Loans	4.15%	4.71%