Short-term and long-term debt - Additional Information (Details) - USD ($)			1 Months Ended										6 Months Ended	12 Months Ended
	Apr. 23, 2020	Dec. 19, 2019	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2021	Dec. 31, 2020	Oct. 10, 2017
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													4.18%	5.84%
Term Loan Facility [Member]
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
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
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
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
Maximum | Term Loan Facility [Member]
Debt Instrument, Interest Rate, Stated Percentage					3.00%								3.00%
Vehicle and Equipment Loans
Line of Credit Facility, Expiration Period													60 months
Vehicle and Equipment Loans | Minimum
Credit facility effective rate of interest					0.00%								0.00%
Vehicle and Equipment Loans | Maximum
Credit facility effective rate of interest					8.30%								8.30%
Term Loan
Debt Instrument, Face Amount															$ 105,000,000
Term Loan | Fourth Amendment
Amendment fee (as a percent)		0.50%
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%
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