Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenues	$ 619,130	$ 519,508	$ 1,181,814	$ 1,020,729
Operating expenses:
Salaries, wages and benefits	196,115	169,543	378,909	330,382
Fuel	65,665	45,129	124,697	90,285
Supplies and maintenance	45,681	40,058	91,420	78,290
Taxes and licenses	22,651	21,638	45,144	42,424
Insurance and claims	30,689	19,827	51,847	39,667
Depreciation	56,551	53,705	112,057	109,041
Rent and purchased transportation	151,433	124,634	287,355	251,059
Communications and utilities	3,928	3,887	8,035	7,959
Other	(4,366)	4,174	(3,548)	8,737
Total operating expenses	568,347	482,595	1,095,916	957,844
Operating income	50,783	36,913	85,898	62,885
Other expense (income):
Interest expense	490	624	972	1,400
Interest income	(693)	(876)	(1,433)	(1,790)
Other	78	152	131	205
Total other income	(125)	(100)	(330)	(185)
Income before income taxes	50,908	37,013	86,228	63,070
Income taxes	12,644	13,794	20,157	23,832
Net income	$ 38,264	$ 23,219	$ 66,071	$ 39,238
Earnings per share:
Basic	$ 0.53	$ 0.32	$ 0.91	$ 0.54
Diluted	0.53	0.32	0.91	0.54
Dividends declared per share	$ 0.090	$ 0.070	$ 0.160	$ 0.130
Weighted-average common shares outstanding:
Basic	72,144	72,227	72,289	72,209
Diluted	72,376	72,492	72,522	72,469