Consolidated Statements Of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenues	$ 2,036,386	$ 2,002,850	$ 1,815,020
Operating expenses:
Salaries, wages and benefits	544,322	536,509	527,576
Fuel	401,417	412,905	313,518
Supplies and maintenance	172,505	169,386	155,943
Taxes and licenses	90,002	92,917	94,018
Insurance and claims	65,593	67,523	69,991
Depreciation	166,957	158,634	152,242
Rent and purchased transportation	420,480	387,472	352,648
Communications and utilities	13,745	15,181	15,123
Other	(10,079)	(11,351)	(621)
Total operating expenses	1,864,942	1,829,176	1,680,438
Operating income	171,444	173,674	134,582
Other expense (income):
Interest expense	288	85	47
Interest income	(1,837)	(1,448)	(1,536)
Other	(173)	131	(166)
Total other income	(1,722)	(1,232)	(1,655)
Income before income taxes	173,166	174,906	136,237
Income taxes	70,132	72,149	56,198
Net income	$ 103,034	$ 102,757	$ 80,039
Earnings per share:
Basic earnings per share	$ 1.41	$ 1.41	$ 1.11
Diluted earnings per share	$ 1.40	$ 1.40	$ 1.10
Weighted-average common shares outstanding:
Basic	72,909	72,787	72,369
Diluted	73,453	73,225	72,807