Loan Participations Sold - Summary of Statistics for Loan Participations Sold (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) Participants	Dec. 31, 2014 USD ($) Participants
Participating Mortgage Loans [Line Items]
Participations Sold, Number of loans | Participants	4	4
Principal balance, Participations Sold	$ 635,581	$ 499,433
Weighted-average cash coupon, Participations Sold	L+2.39 %
Weighted-average all-in cost / yield, Participations Sold	L+2.64 %
Underlying Loans, Number of loans | Participants	4	4
Principal balance, Underlying Loans	$ 786,511	$ 635,701
Weighted-average cash coupon, Underlying Loans	L+4.03 %
Weighted-average all-in cost / yield, Underlying Loans	L+4.26 %
LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon, Participations Sold, rate	2.39%	2.51%
Weighted-average all-in cost / yield, Participations Sold	2.64%	2.71%
Weighted-average cash coupon, Underlying Loans, rate	4.03%	4.10%
Weighted-average all-in cost / yield, Underlying Loans	4.26%	4.71%