Securitized Debt Obligations, Net - Schedule of Information on Securitized Debt Obligations (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Loans	Dec. 31, 2017 USD ($) Loans
Financing Provided [Member]
Debt Instrument [Line Items]
Count | Loans	2	2
Principal Balance	$ 1,292,120	$ 1,292,120
Book Value	$ 1,282,279	$ 1,282,411
Financing Provided [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Wtd.Avg.Yield/Cost	1.82%	1.83%
Financing Provided [Member] | Single Asset Securitization [Member]
Debt Instrument [Line Items]
Count | Loans	1	1
Principal Balance	$ 474,620	$ 474,620
Book Value	$ 474,358	$ 474,328
Term	2033-06	2033-06
Financing Provided [Member] | Single Asset Securitization [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Wtd.Avg.Yield/Cost	1.98%	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	$ 807,921	$ 808,083
Term	2035-06	2035-06
Financing Provided [Member] | Collateralized Loan Obligations [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Wtd.Avg.Yield/Cost	1.73%	1.76%
Collateral Assets [Member]
Debt Instrument [Line Items]
Count | Loans	31	32
Principal Balance	$ 1,668,728	$ 1,656,406
Book Value	$ 1,665,201	$ 1,652,880
Wtd.Avg.Yield/Cost	5.47%	5.17%
Collateral Assets [Member] | Single Asset Securitization [Member]
Debt Instrument [Line Items]
Count | Loans	1	1
Principal Balance	$ 668,728	$ 656,406
Book Value	$ 665,201	$ 652,880
Term	2023-06	2023-06
Collateral Assets [Member] | 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	30	31
Principal Balance	$ 1,000,000	$ 1,000,000
Book Value	$ 1,000,000	$ 1,000,000
Wtd.Avg.Yield/Cost	5.46%	5.16%
Term	2021-11	2021-11