Asset-Backed Financing of a Variable Interest Entity at Fair Value - Summary of Financial Information Relating to Asset-Backed Financing of a VIE at Fair Value (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Variable Interest Entity [Line Items]
Asset-backed financing of a variable interest entity at fair value	$ 270,077		$ 270,077		$ 276,499
Variable Interest Entities [Member]
Variable Interest Entity [Line Items]
Asset-backed financing of a variable interest entity at fair value	270,077		270,077		276,499
Asset-Backed Financing of the VIE at Fair Value [Member] | Variable Interest Entities [Member]
Variable Interest Entity [Line Items]
Asset-backed financing of a variable interest entity at fair value	270,077		270,077		276,499
UPB	268,296		268,296		$ 281,922
Asset-Backed Financing of the VIE at Fair Value [Member] | Variable Interest Entities [Member] | Asset Backed Secured Financing Liability Fair Value [Member]
Variable Interest Entity [Line Items]
Average balance	272,231	$ 289,803	273,999	$ 293,720
Total interest expense	$ 3,557	$ 2,801	$ 6,825	$ 5,097
Weighted average interest rate	3.45%	3.57%	3.50%	3.56%
Weighted average interest rate	3.51%		3.51%		3.51%