Short-term and long-term debt - Additional Information (Details)			1 Months Ended										12 Months Ended
	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 ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019	Oct. 10, 2017 USD ($)
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	6 months
Term Loan Facility [Member]
Debt Instrument, Face Amount															$ 105,000,000
Total Leverage Ratio													5.84%	11.22%
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	$ 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
Debt Instrument, Line Of Credit Facility Periodic Payment Principal		14,000,000
Fourth Amendment | Effective March 31, 2020 [Member]
Principal repayment		562,000
Debt Instrument, Line Of Credit Facility Periodic Payment Principal		562,000
Fourth Amendment | Effective December 31, 2020 [Member]
Principal repayment		1,312,000
Debt Instrument, Line Of Credit Facility Periodic Payment Principal		$ 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 | Term Loan Facility [Member]
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%
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 [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%
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