Consolidated Statements Of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Consolidated Statements Of Income [Abstract]
Operating revenues	$ 498,376	$ 469,429
Operating expenses:
Salaries, wages and benefits	133,848	132,863
Fuel	102,937	97,931
Supplies and maintenance	41,837	41,189
Taxes and licenses	22,532	23,026
Insurance and claims	19,224	18,060
Depreciation	40,671	39,718
Rent and purchased transportation	100,510	88,497
Communications and utilities	3,819	3,923
Other	(2,404)	(3,220)
Total operating expenses	462,974	441,987
Operating income	35,402	27,442
Other expense (income):
Interest expense	142	28
Interest income	(422)	(345)
Other	(24)	26
Total other expense (income)	(304)	(291)
Income before income taxes	35,706	27,733
Income taxes	14,461	11,440
Net income	$ 21,245	$ 16,293
Earnings per share:
Basic	$ 0.29	$ 0.22
Diluted	$ 0.29	$ 0.22
Dividends declared per share	$ 0.050	$ 0.050
Weighted-average common shares outstanding:
Basic	72,854	72,704
Diluted	73,390	73,138