Loan Participations Sold - Summary of Statistics for Loan Participations Sold (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) Loans	Dec. 31, 2015 USD ($) Loans
Participating Mortgage Loans [Line Items]
Participations Sold, Number of loans | Loans	3	3
Principal balance, Participations Sold	$ 478,661	$ 498,992
Book value, Participations Sold	$ 476,894	$ 497,032
Underlying Loans, Number of loans | Loans	3	3
Principal balance, Underlying Loans	$ 583,589	$ 608,554
Book value, Underlying Loans	$ 579,742	$ 604,321
LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon, Participations Sold, rate	2.38%	2.38%
Weighted-average all-in cost / yield, Participations Sold	2.49%	2.49%
Weighted-average cash coupon, Underlying Loans, rate	3.93%	3.93%
Weighted-average all-in cost / yield, Underlying Loans	4.16%	4.15%