Secured Debt Agreements, Net - Summary of Asset-Specific Financings (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan
Participating Mortgage Loans [Line Items]
Financing provided	2023-03	2022-08
Financing provided, Principal Balance	$ 8,816,566	$ 8,996,481
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Count | SecurityLoan	4	1
LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	4.96%	6.08%
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Financing provided	2023-03	2022-08
Guarantee	$ 84,486
Collateral assets, Book Value	314,472	$ 104,807
Financing provided, Book Value	241,597	80,938
Collateral assets, Principal Balance	326,839	106,739
Financing provided, Principal Balance	$ 249,172	$ 81,739
Count | SecurityLoan	4	1
Asset-Specific Financings [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	3.53%	4.07%