Secured Debt Agreements, Net - Summary of Asset-Specific Financings (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Participating Mortgage Loans [Line Items]
Borrowings	$ 518,864	$ 679,207
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Borrowings	518,864	679,207
Cross Collateral Assets [Member] | Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Borrowings	$ 394,800	$ 392,300