Secured Debt Agreements, Net - Summary of Asset-Specific Financings (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan
Participating Mortgage Loans [Line Items]
Financing provided	2022-08	2022-08
Financing provided, Principal Balance	$ 9,235,365	$ 8,996,481
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Count | SecurityLoan	2	1
LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	6.05%	6.08%
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Financing provided	2022-08	2022-08
Collateral assets, Book Value	$ 112,093	$ 104,807
Financing provided, Book Value	91,624	80,938
Collateral assets, Principal Balance	122,699	106,739
Financing provided, Principal Balance	$ 97,699	$ 81,739
Count | SecurityLoan	2	1
Asset-Specific Financings [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	4.06%	4.07%