Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating revenues	$ 2,093,529	$ 2,139,289	$ 2,029,183
Operating expenses:
Salaries, wages and benefits	639,908	584,006	545,419
Fuel	204,583	346,058	371,789
Supplies and maintenance	190,114	188,437	179,172
Taxes and licenses	89,646	85,468	86,686
Insurance and claims	80,848	80,375	71,177
Depreciation	193,209	176,984	173,019
Rent and purchased transportation	480,624	498,782	456,885
Communications and utilities	15,121	14,220	13,506
Other	(980)	4,871	(8,196)
Total operating expenses	1,893,073	1,979,201	1,889,457
Operating income	200,456	160,088	139,726
Other expense (income):
Interest expense	1,974	881	454
Interest income	(2,875)	(2,538)	(2,269)
Other	196	(29)	(170)
Total other income	(705)	(1,686)	(1,985)
Income before income taxes	201,161	161,774	141,711
Income taxes	77,447	63,124	54,926
Net income	$ 123,714	$ 98,650	$ 86,785
Earnings per share:
Basic	$ 1.72	$ 1.37	$ 1.19
Diluted	$ 1.71	$ 1.36	$ 1.18
Weighted-average common shares outstanding:
Basic	71,957	72,122	72,866
Diluted	72,556	72,738	73,428