Secured Debt Agreements, Net - Summary of Asset-Specific Financings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Participating Mortgage Loans [Line Items]
Financing provided(5)	2021-05	2020-07
Collateral assets, Book Value	$ 1,912,990	$ 677,296
Financing provided, Book Value	1,532,306	517,088
Collateral assets, Principal Balance	1,923,404	682,259
Financing provided, Principal Balance	$ 1,538,548	$ 518,864
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Financing provided(5)	2022-09	2020-12
Count | SecurityLoan	5	6
LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	3.49%	4.76%
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Financing provided(5)	2022-09	2020-12
Guarantee	$ 1,172,572	$ 162,475
Financing provided, Principal Balance	$ 1,538,548	$ 518,864
Count | SecurityLoan	5	6
Asset-Specific Financings [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	1.82%	2.50%
GE Portfolio Acquisition Facility [Member]
Participating Mortgage Loans [Line Items]
Guarantee	$ 250,000	$ 250,000
Collateral assets, Book Value	662,521	911,092
Financing provided, Book Value	509,813	702,337
Collateral assets, Principal Balance	660,743	906,707
Financing provided, Principal Balance	$ 510,405	$ 703,423
Count | SecurityLoan	11	16
GE Portfolio Acquisition Facility [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Financing provided(5)	2021-05	2020-07
Count | SecurityLoan	11	16
GE Portfolio Acquisition Facility [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	1.79%	1.72%
GE Portfolio Acquisition Facility [Member] | LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	6.32%	5.74%