Investments - Components of Investment Portfolio, Fair Value (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Fair Value	$ 312,364,636	$ 203,795,813
Percentage of Total Investments	100.00%	100.00%
Agricultural Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 20,351,296	$ 22,851,296
Percentage of Total Investments	6.40%	11.20%
Bulk Fuel Stations and Terminals [Member]
Schedule of Investments [Line Items]
Fair Value	$ 16,259,084	$ 15,437,474
Percentage of Total Investments	5.20%	7.60%
Chemicals and Allied Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 15,000,000
Percentage of Total Investments	4.80%
Coal and Other Minerals and Ores [Member]
Schedule of Investments [Line Items]
Fair Value	$ 30,933,254	$ 6,574,351
Percentage of Total Investments	9.90%	3.20%
Commercial Fishing [Member]
Schedule of Investments [Line Items]
Fair Value	$ 437,814	$ 1,058,273
Percentage of Total Investments	0.10%	0.50%
Communications Equipment [Member]
Schedule of Investments [Line Items]
Fair Value		$ 6,111,941
Percentage of Total Investments	0.00%	3.00%
Consumer Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 11,080,000	$ 9,900,000
Percentage of Total Investments	3.50%	4.90%
Drugs, Proprietaries, and Sundries [Member]
Schedule of Investments [Line Items]
Fair Value	$ 1,080,000
Percentage of Total Investments	0.30%
Electric Services [Member]
Schedule of Investments [Line Items]
Fair Value	$ 11,500,000	$ 19,500,000
Percentage of Total Investments	3.70%	9.60%
Farm Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 2,780,194	$ 3,142,480
Percentage of Total Investments	0.90%	1.50%
Fats and Oils [Member]
Schedule of Investments [Line Items]
Fair Value	$ 12,000,000	$ 6,000,000
Percentage of Total Investments	3.80%	2.90%
Fertilizer & Agricultural Chemicals [Member]
Schedule of Investments [Line Items]
Fair Value		$ 5,078,526
Percentage of Total Investments	0.00%	2.50%
Financial Services [Member]
Schedule of Investments [Line Items]
Fair Value	$ 10,000,000
Percentage of Total Investments	3.20%
Freight Transportation Arrangement [Member]
Schedule of Investments [Line Items]
Fair Value	$ 15,000,000
Percentage of Total Investments	4.80%
Fresh or Frozen Packaged Fish [Member]
Schedule of Investments [Line Items]
Fair Value	$ 2,087,005	$ 5,037,134
Percentage of Total Investments	0.70%	2.50%
Food Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 1,236,274	$ 740,690
Percentage of Total Investments	0.40%	0.40%
Groceries and Related Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 1,476,825	$ 11,195,862
Percentage of Total Investments	0.50%	5.50%
Hotels and Motels [Member]
Schedule of Investments [Line Items]
Fair Value	$ 15,667,791	$ 17,000,000
Percentage of Total Investments	5.00%	8.30%
Land Subdividers and Developers [Member]
Schedule of Investments [Line Items]
Fair Value	$ 14,874,108
Percentage of Total Investments	4.80%
Logging [Member]
Schedule of Investments [Line Items]
Fair Value	$ 3,700,000
Percentage of Total Investments	1.20%
Lumber and Other Construction Materials [Member]
Schedule of Investments [Line Items]
Fair Value		$ 11,483
Percentage of Total Investments	0.00%	0.00%
Meat, Poultry & Fish [Member]
Schedule of Investments [Line Items]
Fair Value	$ 9,000,000	$ 9,675,717
Percentage of Total Investments	2.90%	4.70%
Metals & Mining [Member]
Schedule of Investments [Line Items]
Fair Value	$ 6,566,481	$ 2,234,145
Percentage of Total Investments	2.10%	1.10%
Miscellaneous Plastics Products [Member]
Schedule of Investments [Line Items]
Fair Value		$ 161,018
Percentage of Total Investments	0.00%	0.10%
Packaged Foods & Meats [Member]
Schedule of Investments [Line Items]
Fair Value	$ 500,000	$ 500,000
Percentage of Total Investments	0.20%	0.20%
Personal Credit Institutions [Member]
Schedule of Investments [Line Items]
Fair Value	$ 1,479,786
Percentage of Total Investments	0.50%
Petroleum and Petroleum Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 30,500,000
Percentage of Total Investments	9.80%
Primary Nonferrous Metals [Member]
Schedule of Investments [Line Items]
Fair Value	$ 2,372,297	$ 3,000,000
Percentage of Total Investments	0.80%	1.50%
Primary Metal Industries [Member]
Schedule of Investments [Line Items]
Fair Value	$ 6,000,000	$ 6,000,000
Percentage of Total Investments	1.90%	2.90%
Programming and Data Processing [Member]
Schedule of Investments [Line Items]
Fair Value	$ 14,034,469	$ 10,236,013
Percentage of Total Investments	4.50%	5.00%
Rental of Railroad Cars [Member]
Schedule of Investments [Line Items]
Fair Value	$ 3,513,291	$ 4,411,650
Percentage of Total Investments	1.10%	2.20%
Secondary Nonferrous Metals [Member]
Schedule of Investments [Line Items]
Fair Value	$ 17,349,626	$ 7,649,945
Percentage of Total Investments	5.60%	3.80%
Soap, Detergents, and Cleaning [Member]
Schedule of Investments [Line Items]
Fair Value	$ 1,511,446	$ 2,000,000
Percentage of Total Investments	0.50%	1.00%
Street Construction [Member]
Schedule of Investments [Line Items]
Fair Value	$ 12,218,917	$ 14,927,195
Percentage of Total Investments	3.90%	7.30%
Telephone and Telegraph Apparatus [Member]
Schedule of Investments [Line Items]
Fair Value	$ 8,322,775
Percentage of Total Investments	2.70%
Water Transportation [Member]
Schedule of Investments [Line Items]
Fair Value	$ 13,531,903	$ 13,360,620
Percentage of Total Investments	4.30%	6.60%