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Short-term and long-term debt - Additional Information (Details) - USD ($) |  |  | 1 Months Ended | 3 Months Ended | 9 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 | Sep. 30, 2021 | Sep. 30, 2021 | Dec. 31, 2020 | Oct. 10, 2017
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 |  |  |
TLR [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Leverage Ratio |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.35% | 5.84% |
Term Loan Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent per annum |  |  |  | 10.25% |  |  |  |  |  |  |  |  | 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% | 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% | 0.00% |  |
Vehicle and Equipment Loans | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility effective rate of interest |  |  |  | 8.30% |  |  |  |  |  |  |  |  | 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 |  |

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