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Short-term and long-term debt - Additional Information (Details)					1 Months Ended										3 Months Ended	12 Months Ended
	Apr. 23, 2020 USD ($)	Dec. 30, 2019 USD ($)	Dec. 19, 2019 USD ($)	Oct. 10, 2017 USD ($)	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 ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
TLR [Member]
Total Leverage Ratio															11.28%	11.22%
Amended And Restated Revolving Credit And Security Agreement \| PNC Facility [Member]
Repayments of Lines of Credit			$ 6,700,000
Term Loan Facility [Member]
Amendment fee (as a percent)															0.50%
Fourth Amendment
Principal repayment															$ 14,000,000
Total Leverage Ratio						6.08%	7.03%	7.75%	8.89%	8.89%	10.87%	16.57%	14.54%	15.67%
Minimum Aggregate Undrawn Availability Excess Cash Flow Calculation												$ 5,000,000			$ 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%
Subsequent Event [Member] \| Paycheck Protection Program [Member] \| CARES Act Loan [Member]
Debt Instrument, Interest Rate, Stated Percentage	1.00%
Unsecured loan	$ 2,475,000
Term of forgivable loan	56 days
Term of unforgiven loan	2 years
Deferral term of unforgiven loan	6 years
Subsequent Event [Member] \| First Amendment
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,500,000
Subsequent Event [Member] \| Fifth Amendment To Term Loan Credit And Security Agreement [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,500,000
Vehicle and Equipment Loans
Line of Credit Facility, Expiration Period															60 months
Vehicle and Equipment Loans \| Minimum
Interest rate description under PNC facility															0.0%
Vehicle and Equipment Loans \| Maximum
Interest rate description under PNC facility															8.3%
Term Loan
Debt Instrument, Face Amount				$ 105,000,000
Term Loan \| Euro Dollar
Debt Instrument, Basis Spread on Variable Rate				10.25%
Debt Instrument, Interest Rate, Stated Percentage				3.00%
Term Loan \| Second Amendment
Deferred Financing Costs															$ 1,000,000
Term Loan \| Fourth Amendment
Deferred Financing Costs															$ 500,000
PNC Bank [Member] \| Revolving Credit Facility
Repayments of Lines of Credit																$ 22,300,000
Principal repayment		$ 1,300,000
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%			1.25%
Domestic Rate Loans [Member] \| Maximum \| Amended And Restated Revolving Credit And Security Agreement
Additional interest percentage												1.75%			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%
Wells Fargo
Debt Instrument, Face Amount			15,300,000
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 \| Revolving Credit Facility
Minimum amount to be derived from availability			3,000,000
Minimum aggregate undrawn loan availability			$ 7,500,000
Wells Fargo \| 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