Summary of Significant Accounting Policies - Effect of Adoption of Standard on Condensed Consolidated Statement of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenue		$ 357,010		$ 1,044,505
Purchased transportation	$ 31,216	30,056	$ 95,245	79,516
Total operating expenses		332,442		972,724
Operating income	38,697	24,568	107,841	71,781
Net income	$ 28,195	$ 14,373	$ 79,601	$ 43,339
Basic Earnings Per Share	$ 1.09	$ 0.56	$ 3.09	$ 1.70
Diluted Earnings Per Share	$ 1.07	$ 0.55	$ 3.02	$ 1.67
Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers | As Originally Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenue		$ 350,062		$ 1,025,259
Purchased transportation		23,074		60,212
Total operating expenses		325,460		953,420
Operating income		24,602		71,839
Net income		$ 14,407		$ 43,397
Basic Earnings Per Share		$ 0.56		$ 1.70
Diluted Earnings Per Share		$ 0.55		$ 1.67
Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers | Effect of Change
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenue		$ 6,948		$ 19,246
Purchased transportation		6,982		19,304
Total operating expenses		6,982		19,304
Operating income		(34)		(58)
Net income		$ (34)		$ (58)