Description of Business and Summary of Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Segment Customer	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Description of Business and Summary of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Excess tax benefits related to share-based payments classified as operating activities	$ 1,700
Shares withheld for taxes of financing activities	1,250	$ 650	$ 3,119
Depreciation	85,700	74,500	63,400
Primary payroll-related costs capitalized	$ 2,100	1,600	2,200
Stock-based awards compensation expense amortization period	3 years
Advertising costs	$ 2,200	1,200	1,700
Product Concentration Risk [Member] | Sales Revenue, Net [Member]
Description of Business and Summary of Significant Accounting Policies [Line Items]
Number of customers | Customer	0
Percentage of consolidated operating revenue	5.00%
Restricted Stock [Member]
Description of Business and Summary of Significant Accounting Policies [Line Items]
Stock-based awards compensation expense amortization period	3 years
Trailers [Member]
Description of Business and Summary of Significant Accounting Policies [Line Items]
Assets acquired under capital lease, gross	$ 106,300	72,800
Assets acquired under capital lease, accumulated depreciation	13,300	6,800
Accounting Standards Update 2016-09 [Member]
Description of Business and Summary of Significant Accounting Policies [Line Items]
Excess tax benefits related to share-based payments recorded as offset to income tax expense	1,700
Excess tax benefits related to share-based payments classified as operating activities	1,700
Shares withheld for taxes of financing activities	1,200	700	3,100
Accounting Standards Update 2014-09 [Member]
Description of Business and Summary of Significant Accounting Policies [Line Items]
Retroactive increase in operating revenue and purchased transaction expense	25,800	30,900	30,100
Increase (decrease) in operating revenue	(200)	100	(200)
Increase (decrease) in purchased transportation	$ (100)	$ 100	$ (200)
Minimum [Member]
Description of Business and Summary of Significant Accounting Policies [Line Items]
Percentage of revenue derived from transporting	99.00%
Minimum [Member] | Restricted Stock [Member]
Description of Business and Summary of Significant Accounting Policies [Line Items]
Stock-based awards compensation expense amortization period	3 years
Maximum [Member] | Restricted Stock [Member]
Description of Business and Summary of Significant Accounting Policies [Line Items]
Stock-based awards compensation expense amortization period	5 years