Investments - Components of Investment Portfolio, Fair Value (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Fair Value	$ 330,593,500	$ 340,298,376
Percentage of Total Investments	100.00%	100.00%
Agricultural Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 12,256,800	$ 12,417,122
Percentage of Total Investments	3.70%	3.60%
Boatbuilding and Repairing [Member]
Schedule of Investments [Line Items]
Fair Value	$ 5,848,683	$ 5,695,069
Percentage of Total Investments	1.80%	1.70%
Chemicals and Allied Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 15,000,000	$ 15,000,000
Percentage of Total Investments	4.50%	4.40%
Chocolate and Cocoa Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 9,192,637	$ 9,687,887
Percentage of Total Investments	2.80%	2.80%
Coal and Other Minerals and Ores [Member]
Schedule of Investments [Line Items]
Fair Value	$ 31,959,683	$ 32,348,090
Percentage of Total Investments	9.70%	9.50%
Commercial Fishing [Member]
Schedule of Investments [Line Items]
Fair Value	$ 35,838	$ 35,838
Percentage of Total Investments	0.00%	0.00%
Communications Equipment [Member]
Schedule of Investments [Line Items]
Fair Value		$ 100,000
Percentage of Total Investments		0.00%
Consumer Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 9,318,469	$ 9,318,469
Percentage of Total Investments	2.80%	2.70%
Corrugated And Solid Fiber Boxes [Member]
Schedule of Investments [Line Items]
Fair Value	$ 13,351,735
Percentage of Total Investments	4.00%
Department Stores [Member]
Schedule of Investments [Line Items]
Fair Value	$ 8,786,977	$ 8,638,109
Percentage of Total Investments	2.70%	2.50%
Drugs, Proprietaries, and Sundries [Member]
Schedule of Investments [Line Items]
Fair Value	$ 648,430	$ 803,254
Percentage of Total Investments	0.20%	0.20%
Electric Services [Member]
Schedule of Investments [Line Items]
Fair Value	$ 1,408,638	$ 16,383,269
Percentage of Total Investments	0.40%	4.80%
Farm Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 9,667,785	$ 9,644,313
Percentage of Total Investments	2.90%	2.80%
Fats and Oils [Member]
Schedule of Investments [Line Items]
Fair Value	$ 3,398,558	$ 3,398,558
Percentage of Total Investments	1.00%	1.00%
Financial Services [Member]
Schedule of Investments [Line Items]
Fair Value	$ 3,758,063	$ 3,758,063
Percentage of Total Investments	1.10%	1.10%
Freight Transportation Arrangement [Member]
Schedule of Investments [Line Items]
Fair Value	$ 13,779,518	$ 13,505,035
Percentage of Total Investments	4.20%	4.00%
Food Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 2,242,237	$ 2,724,804
Percentage of Total Investments	0.70%	0.80%
Groceries and Related Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 457,418	$ 468,756
Percentage of Total Investments	0.10%	0.10%
Hotels and Motels [Member]
Schedule of Investments [Line Items]
Fair Value	$ 11,359,874	$ 12,846,584
Percentage of Total Investments	3.40%	3.80%
Land Subdividers and Developers [Member]
Schedule of Investments [Line Items]
Fair Value	$ 14,873,425	$ 16,781,000
Percentage of Total Investments	4.50%	4.90%
Logging [Member]
Schedule of Investments [Line Items]
Fair Value	$ 5,797,622	$ 5,612,436
Percentage of Total Investments	1.80%	1.60%
Meat, Poultry & Fish [Member]
Schedule of Investments [Line Items]
Fair Value	$ 6,361,679	$ 6,240,961
Percentage of Total Investments	1.90%	1.80%
Motor Vehicle Parts and Accessories [Member]
Schedule of Investments [Line Items]
Fair Value	$ 8,974,373	$ 8,731,936
Percentage of Total Investments	2.70%	2.60%
Personal Credit Institutions [Member]
Schedule of Investments [Line Items]
Fair Value	$ 2,975,691	$ 3,603,592
Percentage of Total Investments	0.90%	1.10%
Petroleum and Petroleum Products [Member]
Schedule of Investments [Line Items]
Fair Value	$ 14,592,683	$ 15,500,000
Percentage of Total Investments	4.40%	4.60%
Programming and Data Processing [Member]
Schedule of Investments [Line Items]
Fair Value	$ 17,967,951	$ 17,740,330
Percentage of Total Investments	5.40%	5.20%
Refuse Systems [Member]
Schedule of Investments [Line Items]
Fair Value	$ 27,738,539	$ 25,766,063
Percentage of Total Investments	8.40%	7.60%
Secondary Nonferrous Metals [Member]
Schedule of Investments [Line Items]
Fair Value	$ 17,530,616	$ 17,530,616
Percentage of Total Investments	5.30%	5.20%
Short-Term Business Credit [Member]
Schedule of Investments [Line Items]
Fair Value	$ 4,740,000	$ 4,740,000
Percentage of Total Investments	1.40%	1.40%
Soap, Detergents, and Cleaning [Member]
Schedule of Investments [Line Items]
Fair Value	$ 2,620,083	$ 2,894,698
Percentage of Total Investments	0.80%	0.90%
Telephone and Telegraph Apparatus [Member]
Schedule of Investments [Line Items]
Fair Value	$ 7,686,402	$ 8,840,048
Percentage of Total Investments	2.30%	2.60%
Telephone Communications [Member]
Schedule of Investments [Line Items]
Fair Value	$ 35,747,233	$ 36,794,973
Percentage of Total Investments	10.80%	10.80%
Water Transportation [Member]
Schedule of Investments [Line Items]
Fair Value	$ 10,515,860	$ 12,748,503
Percentage of Total Investments	3.20%	3.70%