Selected Financial Ratios and Other Data - Summary of Selected Financial Ratios and Other Data (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2017
Investment Holdings [Line Items]
Undistributed net investment income per share		$ 0	$ 0
Undistributed net realized gains per share		$ 0	$ 0
Operating expenses		$ 1,150	$ 925	$ 2,092
Total expense ratio	[1],[2]	10.02%	0.10%	2.58%
Operating expense ratio	[1]	5.87%	5.14%	4.16%
Excluding Impact of Medallion Servicing Corp. Amounts [Member]
Investment Holdings [Line Items]
Total expense ratio		11.75%	1.66%	4.25%
Operating expense ratio		6.88%	6.44%	5.64%
Net investment income ratio		7.51%	(1.56%)	(1.22%)
Shareholder Service [Member]
Investment Holdings [Line Items]
Servicing fee		$ 1,290	$ 1,295	$ 2,608

[1] MSC has assumed certain of the Company's servicing obligations, and as a result, servicing fee income of $1,290 and $1,295, and operating expenses of $1,150 and $925, which formerly were the Company's were now MSC's for the three months ended March 31, 2018 and June 30, 2017 and were $2,608 of servicing fee income, and $2,092 of operating expenses for the six months ended June 30, 2017. Excluding the impact of the MSC amounts, the total expense ratio, operating expense ratio, and net investment income ratio would have been 11.75%, 6.88%, and 7.51% in the March 31, 2018 quarter, 1.66%, 6.44%, and (1.56%) in the June 30, 2017 quarter, and 4.25%, 5.64%, and (1.22%) in the six months ended June 30, 2017.

[2] Total expense ratio represents total expenses (interest expense, operating expenses, and income taxes) divided by average net assets.