Short-term and long-term debt - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended	12 Months Ended
	Oct. 10, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Dec. 31, 2017	Apr. 30, 2016	Jun. 22, 2012
Capital leased assets gross				$ 100,000
Line of Credit Facility, Remaining Borrowing Capacity				$ 48,600,000
Earnings Before Interest , Tax ,Depreciation And Amortization		$ 26,100,000	$ 21,900,000
Fixed Charge Coverage Ratio Requirement				1.00 to 1.00
Ratios Of Indebtedness To Net Capital Requirement				4.75 to 1.00
FCCR [Member]
Fixed Charges Coverage Ratio				1.96 to 1	6.47 to 1
TLR [Member]
Fixed Charges Coverage Ratio				10.11 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 | 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				$ 55,000,000
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