Secured Debt Agreements, Net - Summary of Asset-Specific Financings (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($) SecurityLoan
Participating Mortgage Loans [Line Items]
Collateral assets, Book Value	$ 659,152	$ 869,417
Financing provided, Book Value	516,537	676,333
Collateral assets, Principal Balance	662,223	876,083
Financing provided, Principal Balance	$ 517,256	$ 679,207
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2020-12	2020-08
Count | SecurityLoan	5	7
LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	4.70%	4.84%
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2020-12	2020-08
Guarantee	$ 162,517	$ 231,585
Financing provided, Principal Balance	$ 517,256	$ 679,207
Count | SecurityLoan	5	7
Asset-Specific Financings [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	2.48%	2.60%