Short-term and long-term debt - Additional Information (Details)										1 Months Ended											12 Months Ended
	Dec. 19, 2019 USD ($) item	Dec. 03, 2019 USD ($)	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Nov. 30, 2018 USD ($)	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Nov. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018	Jul. 01, 2018	Oct. 10, 2017 USD ($)
Amended And Restated Revolving Credit And Security Agreement | PNC Facility [Member]
Repayments of Lines of Credit	$ 6,700,000
Term Loan Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity																								$ 25,000,000
Debt Instrument, Face Amount																								105,000,000
Percent per annum																		7.25%			7.25%
Total Leverage Ratio																					11.22%	11.82%
Second Amendment
Basis points increase in interest rate																							3.00%
Exit fee percentage																		3.00%			3.00%
One time waiver fee percentage																		1.00%			1.00%
Second Amendment | Repayment Of Loans In Full Occurring Between January 1 2020 And Prior To March 31 2020 [Member]
Reduction in exit fee percentage																		1.50%			1.50%
Third Amendment
Line of Credit Facility, Remaining Borrowing Capacity							$ 28,000,000													$ 28,000,000
Exit fee percentage							5.00%													5.00%
Line of Credit Facility, Periodic Payment, Principal																				$ 525,000
Total Leverage Ratio			6.40%	8.25%	12.95%	12.75%		12.45%	10.15%
Third Amendment | Additional Principal Repayment On Or Before November 14 2019 Minimum Undrawn Availability Greater Than $35000000 [Member]
Percentage Excess Cash Flow							100.00%
Third Amendment | Additional Principal Repayment On Or Before November 14 2019 Minimum Undrawn Availability Greater Than 15000000 Less Than 35000000 [Member]
Percentage Excess Cash Flow							50.00%
Third Amendment | Additional Principal Repayment On Or Before November 14 2019 Minimum Undrawn Availability Less Than 15000000 [Member
Percentage Excess Cash Flow							0.00%
Third Amendment | Voluntary Prepayments Of Loans Made Prior to January 1 2020 [Member]
Reduction in exit fee percentage							0.10%													0.10%
Amount Of Voluntary Prepayments To Which The Exit Fee Reduction Is Applied							$ 1,000,000													$ 1,000,000
Third Amendment | Repayment Of Loans In Full Occurring Between January 1 2020 And Prior To March 31 2020 [Member]
Reduction in exit fee percentage							50.00%													50.00%
Fourth Amendment
Ratios Of Indebtedness To Net Capital										5:36	6.08	7.03	7.75	8.89	8.89	10.87	16.57	14.54	15.67
Principal repayment	14,000,000
Minimum Aggregare Undrawn Availability Excess Cash Flow Calculation	$ 5,000,000
Last twelve months adjusted earnings before interest, tax, depreciation and amortization										$ 16,267,000	$ 15,431,000	$ 14,566,000	$ 14,295,000	$ 12,300,000	$ 12,021,000	$ 11,745,000	$ 7,359,000	$ 7,954,000	$ 7,887,000
Threshold period after trigger event the entity shall commence transaction	6 months
Threshold period after trigger event the entity shall consummate transaction	18 months
Fourth Amendment | Effective March 31, 2020 [Member]
Principal repayment	$ 562,000
Fourth Amendment | Effective December 31, 2020 [Member]
Principal repayment	$ 1,312,000
Fourth Amendment | Through March 31, 2020 [Member]
Amendment fee (as a percent)	0.50%
Fourth Amendment | From April 1, 2020 through March 31, 2021 [Member]
Amendment fee (as a percent)	2.50%
Fourth Amendment | From April 1, 2021 and thereafter [Member]
Amendment fee (as a percent)	5.00%
Vehicle and Equipment Loans
Line of Credit Facility, Expiration Period																					60 months
Minimum | Term Loan Facility [Member]
Exit fee percentage							3.00%													3.00%
Minimum | Third Amendment | Additional Principal Repayment On Or Before November 14 2019 Minimum Undrawn Availability Greater Than 15000000 Less Than 35000000 [Member]
Minimum Aggregare Undrawn Availability Excess Cash Flow Calculation							$ 15,000,000													$ 15,000,000
Minimum | Vehicle and Equipment Loans
Credit facility effective rate of interest																		0.00%			0.00%
Maximum | Additional Principal Repayment On Or Before November 14 2019 Minimum Undrawn Availability Greater Than 15000000 Less Than 35000000 [Member]
Minimum Aggregare Undrawn Availability Excess Cash Flow Calculation							35,000,000													35,000,000
Maximum | Term Loan Facility [Member]
Percentage Excess Cash Flow																					3.00%
Maximum | Third Amendment | Additional Principal Repayment On Or Before November 14 2019 Minimum Undrawn Availability Greater Than $35000000 [Member]
Minimum Aggregare Undrawn Availability Excess Cash Flow Calculation							35,000,000													35,000,000
Maximum | Third Amendment | Additional Principal Repayment On Or Before November 14 2019 Minimum Undrawn Availability Less Than 15000000 [Member
Minimum Aggregare Undrawn Availability Excess Cash Flow Calculation							$ 15,000,000													$ 15,000,000
Maximum | Vehicle and Equipment Loans
Credit facility effective rate of interest																		6.70%			6.70%
Term Loan | Second Amendment
Deferred Financing Costs	$ 1,000,000
Term Loan | Fourth Amendment
Deferred Financing Costs	$ 500,000
Number of independent directors to be appointed by term loan lenders | item	2
Amendment fee (as a percent)	0.50%
PNC Bank [Member] | ABL Loan [Member]
Line of Credit Facility, Maximum Borrowing Capacity																		$ 80,000,000			$ 80,000,000			150,000,000
PNC Bank [Member] | New Revolving Credit Facility
Repayments of Lines of Credit																					22,300,000
Principal repayment																					6,700,000
Domestic Rate Loans [Member] | Amended And Restated Revolving Credit And Security Agreement | London Interbank Offered Rate Daily Libor [Member]
Debt Instrument, Basis Spread on Variable Rate	1.00%
Domestic Rate Loans [Member] | Amended And Restated Revolving Credit And Security Agreement | Federal Funds Purchased [Member]
Debt Instrument, Basis Spread on Variable Rate	0.50%
Domestic Rate Loans [Member] | Minimum | Amended And Restated Revolving Credit And Security Agreement
Additional interest percentage	1.25%
Domestic Rate Loans [Member] | Maximum | Amended And Restated Revolving Credit And Security Agreement
Additional interest percentage	1.75%
Eurodollar Rate Loans [Member] | Minimum | Amended And Restated Revolving Credit And Security Agreement | Euro Dollar
Debt Instrument, Basis Spread on Variable Rate	2.25%
Eurodollar Rate Loans [Member] | Maximum | Amended And Restated Revolving Credit And Security Agreement | Euro Dollar
Debt Instrument, Basis Spread on Variable Rate	2.75%
FS Investment [Member] | Term Loan Facility [Member]
Debt Instrument, Face Amount																								$ 105,000,000
Wells Fargo
Debt Instrument, Face Amount	$ 15,300,000
Principal repayment		$ 1,300,000
Wells Fargo | Amended And Restated Revolving Credit And Security Agreement
Line of Credit Facility, Maximum Borrowing Capacity	60,000,000
Wells Fargo | Swing Line Loan [Member]
Line of Credit Facility, Maximum Borrowing Capacity	5,000,000
Wells Fargo | Letter of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity	2,000,000
Wells Fargo | New Revolving Credit Facility
Minimum amount to be derived from availability	3,000,000
Minimum aggregate undrawn loan availability	$ 7,500,000
Wells Fargo | New Revolving Credit Facility | FCCR [Member]
Fixed Charges Coverage Ratio	1.00
Period for FCCR covenant	2 months
Revolver Guaranty and Security Agreement | Wells Fargo
Right to dominion over certain borrower's bank accounts																					$ 9,000,000