Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues	$ 2,008,991	$ 2,093,529	$ 2,139,289
Operating expenses:
Salaries, wages and benefits	636,112	639,908	584,006
Fuel	155,042	204,583	346,058
Supplies and maintenance	171,397	190,114	188,437
Taxes and licenses	85,547	89,646	85,468
Insurance and claims	83,866	80,848	80,375
Depreciation	209,728	193,209	176,984
Rent and purchased transportation	512,296	480,624	498,782
Communications and utilities	16,106	15,121	14,220
Other	12,827	(980)	4,871
Total operating expenses	1,882,921	1,893,073	1,979,201
Operating income	126,070	200,456	160,088
Other expense (income):
Interest expense	2,577	1,974	881
Interest income	(4,158)	(2,875)	(2,538)
Other	191	196	(29)
Total other income	(1,390)	(705)	(1,686)
Income before income taxes	127,460	201,161	161,774
Income taxes	48,331	77,447	63,124
Net income	$ 79,129	$ 123,714	$ 98,650
Earnings per share:
Basic	$ 1.10	$ 1.72	$ 1.37
Diluted	$ 1.09	$ 1.71	$ 1.36
Weighted-average common shares outstanding:
Basic	72,057	71,957	72,122
Diluted	72,393	72,556	72,738