Long-Term Debt and Financing Arrangements - Additional Information (Detail)		1 Months Ended	12 Months Ended
	Jul. 01, 2017 USD ($)	Feb. 28, 2018 USD ($) Tractor Trailer	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2011 Chassis
Line Of Credit Facility [Line Items]
Number of chassis leased | Chassis						3,126
Chassis, lease agreement period (years)						10 years
Capital lease interest paid			$ 500,000	$ 600,000	$ 700,000
Standby Letters of Credit [Member]
Line Of Credit Facility [Line Items]
Letters of credit expiration, year			2018
Outstanding letters of credit			$ 20,100,000
Revolving Line of Credit Facility [Member] | Bank Revolving Line of Credit [Member]
Line Of Credit Facility [Line Items]
Unused and available borrowings under bank revolving line of credit and credit agreement			$ 284,900,000
Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Amount of credit agreement	$ 350,000,000
Credit agreement interest rate description			(i) LIBOR plus a specified margin based upon the Borrowers' total net leverage ratio (as defined in the Credit Agreement) (the "Total Net Leverage Ratio"), or (ii) the base rate (which is the highest of (a) the administrative agent's prime rate, (b) the federal funds rate plus 0.50% or (c) the sum of 1% plus one-month LIBOR) plus a specified margin based upon the Total Net Leverage Ratio.
Commitment fee on unused commitments	0.25%
Credit Agreement covenant terms			The Credit Agreement contains various restrictions and covenants, including negative covenants that limit or restrict dividends, indebtedness of subsidiaries, mergers and fundamental changes, asset sales, acquisitions, liens and encumbrances, transactions with affiliates, changes in fiscal year and other matters customarily restricted in such agreements. The Company must maintain a Total Net Leverage Ratio of (a) total funded debt as of such date, minus up to $50.0 million in unrestricted cash and cash equivalents (each as defined in the Credit Agreement) to (b) consolidated EBITDA (as defined in the credit agreement) of not more than 3.00 to 1.00; provided that as of the close of each of the four fiscal quarters occurring after the consummation of a permitted acquisition (as defined in the Credit Agreement) with an aggregate consideration of $150.0 million or more, such ratio shall not be more than 3.25 to 1.00. The Company must maintain an interest coverage ratio of consolidated EBITDA to consolidated cash interest expense of not less than 3.00 to 1.00.
Debt covenant permitted aggregate consideration	$ 150,000,000
Credit Agreement [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Restricted cash and cash equivalents	$ 50,000,000
Total leverage ratio	3.00%
Total leverage ratio for following four each fiscal quarters	3.25%
Credit Agreement [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Interest coverage ratio	3.00%
Credit Agreement [Member] | Federal Funds Rate [Member]
Line Of Credit Facility [Line Items]
Interest rate under the credit agreement	0.50%
Credit Agreement [Member] | LIBOR rate [Member]
Line Of Credit Facility [Line Items]
Interest rate under the credit agreement	1.00%
Description of variable rate basis	one-month LIBOR
Credit Agreement [Member] | Eurodollar
Line Of Credit Facility [Line Items]
Interest rate under the credit agreement	2.00%
Credit Agreement [Member] | Base Rate
Line Of Credit Facility [Line Items]
Interest rate under the credit agreement	1.00%
Letter of Credit [Member]
Line Of Credit Facility [Line Items]
Commitment fee on unused commitments	2.00%
2013 Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Unused and available borrowings under bank revolving line of credit and credit agreement				$ 38,200,000
Secured Equipment Note [Member] | Subsequent Event [Member]
Line Of Credit Facility [Line Items]
Purchase contract units tractors | Tractor		29
Purchase contract units trailers | Trailer		36
Cost of purchasing equipment		$ 5,000,000
Payment Frequency		monthly
Debt instrument, payment terms		The note calls for 60 monthly payments
Monthly payments		$ 91,986
Fixed interest rate		3.56%