Secured Financings - Summary of Loan Participations Sold (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Participants	Dec. 31, 2013 Participants
Participating Mortgage Loans [Line Items]
Participations sold, Number of loans	4	1
Principal balance, Participations sold	$ 499,433	$ 90,000
Weighted-average cash coupon, Participations sold	L+2.51 %	L+5.12 %
Weighted-average all-in yield / cost, Participations sold	L+2.71 %	L+5.26 %
Underlying Loans, Number of loans	4	1
Principal balance, Underlying Loans	$ 635,701	$ 173,837
Weighted-average cash coupon, Underlying Loans	L+4.10 %	L+5.66 %
Weighted-average all-in yield / cost, Underlying Loans	L+4.71 %	L+9.25 %
LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon, Participations sold, rate	2.51%	5.12%
Weighted-average all-in yield / cost, Participations sold, rate	2.71%	5.26%
Weighted-average cash coupon, Underlying Loans, rate	4.10%	5.66%
Weighted-average all-in yield / cost, Underlying Loans, rate	4.71%	9.25%