Secured Debt Agreements, Net - Summary of Asset-Specific Financings (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Participating Mortgage Loans [Line Items]
Collateral assets, Book Value	$ 2,367,275	$ 677,296
Financing provided, Book Value	1,878,776	517,088
Collateral assets, Principal Balance	2,384,093	682,259
Financing provided, Principal Balance	$ 1,885,863	$ 518,864
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2021-11	2020-12
Count | SecurityLoan	6	6
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2021-11	2020-12
Guarantee	$ 1,392,274	$ 162,475
Financing provided, Principal Balance	$ 1,885,863	$ 518,864
Count | SecurityLoan	6	6
Asset-Specific Financings [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	1392274.00%	162475.00%
GE Portfolio Acquisition Facility [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2021-05	2020-07
Guarantee	$ 250,000	$ 250,000
Collateral assets, Book Value	646,554	911,092
Financing provided, Book Value	508,615	702,337
Collateral assets, Principal Balance	645,107	906,707
Financing provided, Principal Balance	$ 509,441	$ 703,423
Count | SecurityLoan	11	16
GE Portfolio Acquisition Facility [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	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	1.78%	1.72%
GE Portfolio Acquisition Facility [Member] | LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	5.99%	5.74%