Secured Financings - Summary of Statistics for Asset-Specific Repurchase Agreements (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Loans	Dec. 31, 2013 Loans
Assets Sold Under Agreements To Repurchase Carrying Amounts [Member]
Participating Mortgage Loans [Line Items]
Number of loans	3	4
Principal balance	$ 324,553	$ 245,731
Weighted-average cash coupon	L+2.68 %	L+2.55 %
Weighted-average all-in yield / cost	L+3.16 %	L+3.03 %
Assets Sold Under Agreements To Repurchase Carrying Amounts [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon	2.68%	2.55%
Weighted-average all-in yield / cost	3.16%	3.03%
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Number of loans	4	4
Principal balance	$ 429,197	$ 334,857
Weighted-average cash coupon	L+5.07 %	L+4.79 %
Weighted-average all-in yield / cost	L+5.53 %	L+5.38 %
Collateral Assets [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted-average cash coupon	5.07%	4.79%
Weighted-average all-in yield / cost	5.53%	5.38%