Financial Highlights - Schedule of Financial Highlights (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Net asset value at beginning of period			$ 8.20	$ 8.42
Net investment income	$ 0.12	$ 0.15	0.25	0.29
Net change in unrealized depreciation on investments			(0.04)	(0.09)
Net increase in net assets resulting from operations			0.21	0.20
Distributions			(0.30)	(0.32)
Net change in accrued distribution and other fees			0.01	0.02
Net decrease in net assets			(0.08)	(0.11)
Net asset value at end of period	$ 8.11	$ 8.31	$ 8.11	$ 8.31
Total return based on net asset value			2.57%	2.34%
Net assets at end of period	$ 353,770,077	$ 366,614,079	$ 353,770,077	$ 366,614,079	$ 360,070,359	$ 321,356,737
Units Outstanding at end of period	43,602,953	44,080,551	43,602,953	44,080,551	43,914,946
Ratio/Supplemental data (annualized):
Ratio of net investment income to average net assets			6.31%	7.04%
Ratio of net operating expenses to average net assets			6.33%	5.61%