Secured Debt Agreements, Net - Summary of Asset-Specific Financings (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan
Participating Mortgage Loans [Line Items]
Financing provided	2023-03	2022-08
Financing provided, Principal Balance	$ 8,283,613	$ 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	4.94%	6.08%
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Financing provided	2023-03	2022-08
Guarantee	$ 84,547
Collateral assets, Book Value	268,864	$ 104,807
Financing provided, Book Value	209,912	80,938
Collateral assets, Principal Balance	281,620	106,739
Financing provided, Principal Balance	$ 217,938	$ 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%