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
	Mar. 31, 2018	Mar. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenue		$ 323,090
Purchased transportation	$ 29,916	20,820
Total operating expenses	365,226	305,563
Operating income	27,579	17,527
Net income	$ 21,125	$ 11,395
Basic Earnings Per Share	$ 0.82	$ 0.45
Diluted Earnings Per Share	$ 0.80	$ 0.44
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		$ 317,037
Purchased transportation		14,775
Total operating expenses		299,518
Operating income		17,519
Net income		$ 11,387
Basic Earnings Per Share		$ 0.45
Diluted Earnings Per Share		$ 0.44
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		$ 6,053
Purchased transportation		6,045
Total operating expenses		6,045
Operating income		8
Net income		$ 8