Short-term and long-term debt - Additional Information (Details)				1 Months Ended										3 Months Ended	12 Months Ended
	Mar. 02, 2022 USD ($)	Apr. 23, 2020 USD ($)	Dec. 19, 2019 USD ($) item	Dec. 31, 2021 USD ($)	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 ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020	Mar. 02, 2027	Oct. 10, 2017 USD ($)
Forgiveness of loan															$ (2,475,000)
Paycheck Protection Program [Member] | CARES Act Loan [Member]
Debt Instrument, Interest Rate, Stated Percentage		1.00%
Unsecured loan		$ 2,475,000
Term of unforgiven loan		2 years
Deferral term of unforgiven loan		9 months
Forgiveness of loan														$ 2,475,000
Prior Term Loan Facility
Debt Instrument, Face Amount																		$ 105,000,000
Total Leverage Ratio															1.93%	5.84%
Percent per annum				10.25%											10.25%
First Amendment
Line of Credit Facility, Maximum Borrowing Capacity		$ 2,500,000
Fourth Amendment
Principal repayment			$ 14,000,000
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
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	$ 12,021,000.000	$ 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
Minimum Aggregate Undrawn Availability Excess Cash Flow Calculation			$ 5,000,000
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%
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
Minimum | Vehicle and Equipment Loans
Credit facility effective rate of interest				0.00%											0.00%
Maximum | Prior Term Loan Facility
Term Loan Facility- net of current portion of long-term debt															3.00%
Maximum | Vehicle and Equipment Loans
Credit facility effective rate of interest				8.30%											8.30%
Subsequent Event [Member]
Debt Instrument, Basis Spread on Variable Rate	2.00%
Subsequent Event [Member] | Federal Funds Purchased [Member]
Debt Instrument, Basis Spread on Variable Rate	0.50%
Subsequent Event [Member] | Prior Term Loan Facility
Debt Instrument, Face Amount	$ 85,000,000
Prepayment premium percentage year two																	2.00%
Prepayment premium percentage year three																	1.00%
Term Loan | Fourth Amendment
Amendment fee (as a percent)			0.50%
Deferred Financing Costs			$ 500,000
Number of independent directors to be appointed by term loan lenders | item			2
Domestic rate loans | 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 | Amended And Restated Revolving Credit And Security Agreement | Federal Funds Purchased [Member]
Debt Instrument, Basis Spread on Variable Rate			0.50%
Domestic rate loans | Minimum | Amended And Restated Revolving Credit And Security Agreement
Additional interest percentage			1.25%
Domestic rate loans | 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%
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
Wells Fargo | Subsequent Event [Member]
Line of Credit Facility, Maximum Borrowing Capacity	90,000,000
Wells Fargo | Subsequent Event [Member] | FCCR [Member]
Minimum aggregate undrawn loan availability	$ 11,250,000
Period for FCCR covenant	12 months
Wells Fargo | Subsequent Event [Member] | Amended And Restated Revolving Credit And Security Agreement
Line of Credit Facility, Maximum Borrowing Capacity	$ 75,000,000
Wells Fargo | Subsequent Event [Member] | Swing Line Loan [Member]
Line of Credit Facility, Maximum Borrowing Capacity	9,000,000
Wells Fargo | Subsequent Event [Member] | Letter of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity	2,000,000
Wells Fargo | Subsequent Event [Member] | Minimum
Minimum amount to be derived from availability	$ 5,000,000
Wells Fargo | Subsequent Event [Member] | Minimum | FCCR [Member]
Fixed Charges Coverage Ratio	1.00
Wells Fargo | Subsequent Event [Member] | Maximum
Minimum amount to be derived from availability	$ 3,000,000
Wells Fargo | Subsequent Event [Member] | Maximum | FCCR [Member]
Fixed Charges Coverage Ratio	1.00
Revolver Guaranty and Security Agreement | Wells Fargo
Right to dominion over certain borrower's bank accounts															$ 9,000,000