Secured Financings - Summary of Statistics for Asset-Specific Repurchase Agreements (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015 Loans	Dec. 31, 2014 Loans
Assets Sold Under Agreements To Repurchase Carrying Amounts [Member]
Participating Mortgage Loans [Line Items]
Number of loans	4	3
Principal balance	$ 407,203	$ 324,553
Weighted-average cash coupon	L+2.75 %	L+2.68 %
Weighted-average cost / all-in yield	L+3.20 %	L+3.16 %
Assets Sold Under Agreements To Repurchase Carrying Amounts [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon	2.75%	2.68%
Weighted-average cost / all-in yield	3.20%	3.16%
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Number of loans	5	4
Principal balance	$ 539,043	$ 429,197
Weighted-average cash coupon	L+5.21 %	L+5.07 %
Weighted-average cost / all-in yield	L+5.70 %	L+5.53 %
Collateral Assets [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon	5.21%	5.07%
Weighted-average cost / all-in yield	5.70%	5.53%