Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating revenues	$ 2,457,914	$ 2,116,737	$ 2,008,991
Operating expenses:
Salaries, wages and benefits	781,064	681,547	636,112
Fuel	254,564	198,745	155,042
Supplies and maintenance	185,074	164,325	171,397
Taxes and licenses	87,318	86,768	85,547
Insurance and claims	98,133	79,927	83,866
Depreciation	230,151	217,639	209,728
Rent and purchased transportation	589,002	509,573	512,296
Communications and utilities	16,063	16,105	16,106
Other	(7,670)	18,288	12,827
Total operating expenses	2,233,699	1,972,917	1,882,921
Operating income	224,215	143,820	126,070
Other expense (income):
Interest expense	2,695	2,243	2,577
Interest income	(2,737)	(3,308)	(4,158)
Other	376	328	191
Total other expense (income)	334	(737)	(1,390)
Income before income taxes	223,881	144,557	127,460
Income tax expense (benefit)	55,733	(58,332)	48,331
Net income	$ 168,148	$ 202,889	$ 79,129
Earnings per share:
Basic	$ 2.35	$ 2.81	$ 1.10
Diluted	$ 2.33	$ 2.80	$ 1.09
Weighted-average common shares outstanding:
Basic	71,694	72,270	72,057
Diluted	72,057	72,558	72,393