Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues	$ 2,116,737	$ 2,008,991	$ 2,093,529
Operating expenses:
Salaries, wages and benefits	681,547	636,112	639,908
Fuel	198,745	155,042	204,583
Supplies and maintenance	164,325	171,397	190,114
Taxes and licenses	86,768	85,547	89,646
Insurance and claims	79,927	83,866	80,848
Depreciation	217,639	209,728	193,209
Rent and purchased transportation	509,573	512,296	480,624
Communications and utilities	16,105	16,106	15,121
Other	18,288	12,827	(980)
Total operating expenses	1,972,917	1,882,921	1,893,073
Operating income	143,820	126,070	200,456
Other expense (income):
Interest expense	2,243	2,577	1,974
Interest income	(3,308)	(4,158)	(2,875)
Other	328	191	196
Total other income	(737)	(1,390)	(705)
Income before income taxes	144,557	127,460	201,161
Income tax expense (benefit)	(58,332)	48,331	77,447
Net income	$ 202,889	$ 79,129	$ 123,714
Earnings per share:
Basic	$ 2.81	$ 1.10	$ 1.72
Diluted	$ 2.80	$ 1.09	$ 1.71
Weighted-average common shares outstanding:
Basic	72,270	72,057	71,957
Diluted	72,558	72,393	72,556