Description of Business and Summary of Accounting Policies - Effect of Adoption of Standard on Condensed Consolidated Statement of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenue	$ 406,750	$ 425,562	$ 428,732	$ 392,805	$ 360,196	$ 357,010	$ 364,404	$ 323,090	$ 1,653,849	$ 1,404,703	$ 1,250,391
Purchased transportation					28,185				123,904	107,702	88,239
Total operating expenses					337,268					1,309,993	1,171,255
Operating income	33,336	38,697	41,565	27,579	22,928	24,568	29,686	17,527	141,177	94,710	79,136
Net income	$ 25,380	$ 28,195	$ 30,281	$ 21,125	$ 47,789	$ 14,373	$ 17,571	$ 11,395	$ 104,981	$ 91,129	$ 48,024
Basic Earnings Per Share	$ 0.98	$ 1.09	$ 1.18	$ 0.82	$ 1.87	$ 0.56	$ 0.69	$ 0.45	$ 4.08	$ 3.57	$ 1.92
Diluted Earnings Per Share	$ 0.97	$ 1.07	$ 1.15	$ 0.80	$ 1.82	$ 0.55	$ 0.68	$ 0.44	$ 3.99	$ 3.49	$ 1.87
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					$ 353,251					$ 1,378,510	$ 1,218,481
Purchased transportation					21,270					81,482	56,329
Total operating expenses					330,353					1,283,773	1,139,345
Operating income					22,898					94,737	79,136
Net income					$ 47,759					$ 91,156	$ 48,024
Basic Earnings Per Share					$ 1.87					$ 3.57	$ 1.92
Diluted Earnings Per Share					$ 1.82					$ 3.49	$ 1.87
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					$ 6,945					$ 26,193	$ 31,910
Purchased transportation					6,915					26,220	31,910
Total operating expenses					6,915					26,220	$ 31,910
Operating income					30					(27)
Net income					$ 30					$ (27)