Secured Debt Agreements, Net - Summary of Asset-Specific Financings (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Participating Mortgage Loans [Line Items]
Collateral assets, Book Value	$ 2,060,036	$ 677,296
Financing provided, Book Value	1,600,256	517,088
Collateral assets, Principal Balance	2,075,003	682,259
Financing provided, Principal Balance	$ 1,607,424	$ 518,864
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2022-07	2020-12
Count | SecurityLoan	7	6
LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	3.82%	4.76%
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2022-07	2020-12
Guarantee	$ 1,229,828	$ 162,475
Financing provided, Principal Balance	$ 1,607,424	$ 518,864
Count | SecurityLoan	7	6
Asset-Specific Financings [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	1.76%	2.50%
GE Portfolio Acquisition Facility [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2021-03	2020-07
Guarantee	$ 250,000	$ 250,000
Collateral assets, Book Value	711,057	911,092
Financing provided, Book Value	558,670	702,337
Collateral assets, Principal Balance	709,644	906,707
Financing provided, Principal Balance	$ 559,637	$ 703,423
Count | SecurityLoan	13	16
GE Portfolio Acquisition Facility [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2021-03	2020-07
Count | SecurityLoan	13	16
GE Portfolio Acquisition Facility [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	1.69%	1.72%
GE Portfolio Acquisition Facility [Member] | LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	5.91%	5.74%