Short-term and long-term debt - Additional Information (Detail) - USD ($)			1 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended
	Aug. 14, 2018	Oct. 10, 2017	Nov. 30, 2018	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Dec. 31, 2017	Apr. 30, 2016	Jun. 22, 2012
Capital leased assets gross						$ 100,000
Debt Instrument, Payment Terms			the Third Amendment requires a further repayment of principal on or before November 14, 2019 in an amount equal to (x) 100% of Excess Cash Flow (as defined in the Term Loan Facility) for the four fiscal quarter period ending September 30, 2019 if after giving effect to the payment thereof, the Borrowers have minimum aggregate Undrawn Availability (as defined in the Term Loan Facility) of at least $35,000,000, (y) 50% of Excess Cash Flow for the four fiscal quarter period ending September 30, 2019 if after giving effect to the payment thereof, the Borrowers have minimum aggregate Undrawn Availability of at least $15,000,000 but less than $35,000,000, and (z) 0% of Excess Cash Flow for the four fiscal quarter period ending September 30, 2019 if after giving effect to the payment thereof, the Borrowers have minimum aggregate Undrawn Availability less than $15,000,000, with any such payment subject to reduction by the amount of any voluntary prepayments made following the date of the Third Amendment.
Line of Credit Facility, Remaining Borrowing Capacity						$ 43,500,000
Earnings Before Interest , Tax ,Depreciation And Amortization				$ 26,100,000	$ 21,900,000
Ratios Of Indebtedness To Net Capital Requirement						4.75 to 1.00
Deferred Tax Liabilities, Deferred Expense, Debt Issuance Costs						$ 1,000,000
FCCR [Member]
Fixed Charge Coverage Ratio Requirement						1.00 to 1.00
Fixed Charges Coverage Ratio						1.74 to 1	6.47 to 1
TLR [Member]
Ratios Of Indebtedness To Net Capital						12.44 to 1	3.03 to 1
Revolving Credit Facility
Line of Credit Facility, Maximum Borrowing Capacity									$ 46,000,000
Minimum
Line of Credit Facility, Maximum Borrowing Capacity									$ 40,000,000
Maximum
Line of Credit Facility, Maximum Borrowing Capacity								$ 50,000,000
Vehicle and Equipment Loans | Minimum
Credit facility effective rate of interest						0.00%
Vehicle and Equipment Loans | Maximum
Credit facility effective rate of interest						6.70%
Term Loan
Line of Credit Facility, Maximum Borrowing Capacity		$ 25,000,000
Interest rate description under PNC facility						Interest charges with respect to loans are computed on the actual principal amount of loans outstanding during the month at a rate per annum equal to (A) with respect to domestic rate loans, the sum of (i) a rate per annum equal to the higher of (1) the base commercial lending rate of PNC, (2) the federal funds open rate plus 0.5% and (3) the daily LIBOR plus 1.0%, plus (ii) between 0.50% and 1.00% depending on average quarterly undrawn availability and (B) with respect to Eurodollar rate loans, the sum of the Eurodollar rate plus between 1.50% and 2.00% depending on average quarterly undrawn availability.
Debt Instrument, Face Amount		$ 105,000,000
Debt Instrument, Payment Terms		Principal payments on the Term Loans are required on a quarterly basis, commencing with the quarter ended March 31, 2018, in the amount of 1% per annum of the original principal of the outstanding Term Loans. Commencing with the fiscal year ending December 31, 2018, the Term Loan Facility also requires annual principal payments of up to 50% of Excess Cash Flow (as defined in the Term Loan Facility) if the Company’s Total Leverage Ratio (as defined in the Term Loan Facility) for the applicable year is greater than 2.75 to 1.00. The Term Loan Facility also requires mandatory prepayments of the Term Loans in the event of certain asset dispositions, debt issuances, and casualty and condemnation events. The Term Loans may be prepaid at the option of the Borrowers at par in an amount up to $30 million. Additional prepayments are permitted after the first anniversary of the closing date and were originally subject to a prepayment premium of 3% in year two, 1% in year three and zero in year four and thereafter.
Term Loan | Second Amendment [Member]
Debt Instrument Amendment New Covenant Description	(the “Second Amendment”) with U.S. Bank National Association, as collateral agent and administrative agent, and the various lenders thereunder. The Second Amendment superseded interim waivers and amended the Term Loan Facility, to waive compliance with the existing TLR covenant at June 30, 2018.
Debt Instrument, Covenant Compliance	(i) increases the interest rate by 300 basis points effective July 1, 2018; (ii) waives the existing prepayment premium in the Term Loan Facility in the event the term loan is repaid in full prior to March 31, 2020; (iii) adds an exit fee equal to three percent (3.00%) of the outstanding principal balance of the term loans on the date of the Second Amendment (provided, that payment of the exit fee is waived in the event that the term loan is repaid in full prior to January 1, 2020, and provided further that the exit fee is reduced to one-and-one-half percent (1.50%) in the event that the term loan is repaid in full on or after January 1, 2020 but prior to March 31, 2020); (iv) restricts acquisitions and other equity investments prior to September 30, 2018; and (v) required payment of a one-time waiver fee equal to one percent (1.00%) of the outstanding term loans.
Term Loan | Eurodollar
Debt Instrument, Basis Spread on Variable Rate		7.25%
Debt Instrument, Interest Rate, Stated Percentage		3.00%
PNC Bank [Member]
Line of Credit Facility, Maximum Borrowing Capacity		$ 150,000,000				$ 80,000,000
Letters of Credit Outstanding, Amount					$ 130,000	130,000
Debt Instrument, Face Amount						$ 29,000,000
PNC Bank [Member] | Letter of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity		5,000,000
PNC Bank [Member] | Swing Line Loan [Member]
Line of Credit Facility, Maximum Borrowing Capacity		$ 15,000,000
Busey Bank
Line of Credit Facility, Expiration Period						60 months