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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenue		$ 364,404		$ 687,494
Purchased transportation	$ 34,113	28,639	$ 64,029	49,459
Total operating expenses		334,718		640,281
Operating income	41,565	29,686	69,144	47,213
Net income	$ 30,281	$ 17,571	$ 51,406	$ 28,966
Basic Earnings Per Share	$ 1.18	$ 0.69	$ 2.00	$ 1.14
Diluted Earnings Per Share	$ 1.15	$ 0.68	$ 1.95	$ 1.12
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		$ 358,160		$ 675,197
Purchased transportation		22,363		37,138
Total operating expenses		328,442		627,960
Operating income		29,718		47,237
Net income		$ 17,603		$ 28,990
Basic Earnings Per Share		$ 0.69		$ 1.14
Diluted Earnings Per Share		$ 0.68		$ 1.12
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		$ 6,244		$ 12,297
Purchased transportation		6,276		12,321
Total operating expenses		6,276		12,321
Operating income		(32)		(24)
Net income		$ (32)		$ (24)