Short-term and long-term debt - Additional Information (Details)				1 Months Ended										6 Months Ended	9 Months Ended	12 Months Ended
	Apr. 23, 2020 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 ($)	Jun. 30, 2020	Sep. 30, 2020 USD ($)	Dec. 31, 2019
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
TLR [Member]
Total Leverage Ratio															5.79%	11.22%
Term Loan Facility [Member]
Amendment fee (as a percent)															0.50%
First Amendment
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,500,000
Fourth Amendment
Principal repayment															$ 14,000,000
Total Leverage Ratio				5.36%	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
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
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 | Fourth Amendment
Deferred Financing Costs															$ 500,000
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 | 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]
Period for FCCR covenant		2 months
Wells Fargo | Minimum | Revolving Credit Facility | FCCR [Member]
Fixed Charges Coverage Ratio		1.00
Wells Fargo | Maximum | Revolving Credit Facility | 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