Securitized Debt Obligations, Net - Schedule of Information on Securitized Debt Obligations (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Loans	Dec. 31, 2017 USD ($) Loans	Jul. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)
Debt Instrument [Line Items]
Principal Balance			$ 1,000,000	$ 474,600
Financing Provided [Member]
Debt Instrument [Line Items]
Count | Loans	2	2
Principal Balance	$ 1,292,120	$ 1,292,120
Book Value	$ 1,284,362	$ 1,282,411
Financing Provided [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Wtd. Avg. Yield/Cost	1.71%	1.83%
Financing Provided [Member] | 2017 Single Asset Securitization [Member]
Debt Instrument [Line Items]
Count | Loans	1	1
Principal Balance	$ 474,620	$ 474,620
Book Value	$ 474,418	$ 474,328
Term	2033-06	2033-06
Financing Provided [Member] | 2017 Single Asset Securitization [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Wtd. Avg. Yield/Cost	1.65%	1.94%
Financing Provided [Member] | Collateralized Loan Obligations [Member]
Debt Instrument [Line Items]
Count | Loans	1	1
Principal Balance	$ 817,500	$ 817,500
Book Value	$ 809,944	$ 808,083
Term	2035-06	2035-06
Financing Provided [Member] | Collateralized Loan Obligations [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Wtd. Avg. Yield/Cost	1.74%	1.76%
Collateral Assets [Member]
Debt Instrument [Line Items]
Count | Loans	29	32
Principal Balance	$ 1,678,963	$ 1,656,406
Book Value	$ 1,675,437	$ 1,652,880
Wtd. Avg. Yield/Cost	6.00%	5.17%
Collateral Assets [Member] | 2017 Single Asset Securitization [Member]
Debt Instrument [Line Items]
Count | Loans	1	1
Principal Balance	$ 678,963	$ 656,406
Book Value	$ 675,437	$ 652,880
Term	2023-06	2023-06
Collateral Assets [Member] | 2017 Single Asset Securitization [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Wtd. Avg. Yield/Cost	3.60%	3.60%
Collateral Assets [Member] | Collateralized Loan Obligations [Member]
Debt Instrument [Line Items]
Count | Loans	28	31
Principal Balance	$ 1,000,000	$ 1,000,000
Book Value	$ 1,000,000	$ 1,000,000
Wtd. Avg. Yield/Cost	6.09%	5.16%
Term	2022-02	2021-11