Secured Financings - Summary of Loan Participations Sold (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015 Participants	Dec. 31, 2014 Participants
Participating Mortgage Loans [Line Items]
Participations sold, Number of loans	4	4
Principal balance, Participations sold	$ 708,845	$ 499,433
Weighted-average cash coupon, Participations sold	L+2.40 %	L+2.51 %
Weighted-average all-in cost / yield, Participations sold	L+2.68 %	L+2.71 %
Underlying Loans, Number of loans	4	4
Principal balance, Underlying Loans	$ 879,043	$ 635,701
Weighted-average cash coupon, Underlying Loans	L+4.06 %	L+4.10 %
Weighted-average all-in cost / yield, Underlying Loans	L+4.36 %	L+4.71 %
LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon, Participations sold, rate	2.40%	2.51%
Weighted-average all-in cost / yield, Participations sold	2.68%	2.71%
Weighted-average cash coupon, Underlying Loans, rate	4.06%	4.10%
Weighted-average all-in cost / yield, Underlying Loans	4.36%	4.71%