Secured Debt Agreements - Summary of Statistics for Asset-Specific Repurchase Agreements (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) Loans	Dec. 31, 2014 USD ($) Loans
Participating Mortgage Loans [Line Items]
Book value	$ 6,116,105	$ 2,353,279
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Number of loans | Loans	2	4
Principal balance	$ 154,192	$ 429,197
Book value	$ 153,542	$ 425,680
Collateral Assets [Member] | Weighted-Average Cash Coupon [Member]
Participating Mortgage Loans [Line Items]
Debt instrument variable rate basis	L+5.27 %
Weighted-average cash coupon, Basis spread on variable rate	5.27%	5.07%
Collateral Assets [Member] | Weighted-Average All-in Cost of Credit [Member]
Participating Mortgage Loans [Line Items]
Debt instrument variable rate basis	L+5.83 %
Weighted-average cost / all-in yield, rate	5.83%	5.53%
Asset-Specific Repurchase Agreements [Member]
Participating Mortgage Loans [Line Items]
Number of loans | Loans	2	3
Principal balance	$ 111,746	$ 324,553
Book value	$ 111,061	$ 321,347
Asset-Specific Repurchase Agreements [Member] | Weighted-Average Cash Coupon [Member]
Participating Mortgage Loans [Line Items]
Debt instrument variable rate basis	L+2.52 %
Weighted-average cash coupon, Basis spread on variable rate	2.52%	2.68%
Asset-Specific Repurchase Agreements [Member] | Weighted-Average All-in Cost of Credit [Member]
Participating Mortgage Loans [Line Items]
Debt instrument variable rate basis	L+2.88 %
Weighted-average cost / all-in yield, rate	2.88%	3.16%