Condensed Statements Of Operations (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Statement [Abstract]
Revenue from operations	$ 590,304	$ 547,452	$ 1,128,720	$ 1,050,271
Operating expenses:
Salaries, wages and benefits	287,961	268,502	558,761	526,491
Operating supplies and expenses	95,115	93,691	190,818	187,907
General supplies and expenses	17,639	15,937	35,400	30,089
Operating taxes and licenses	17,982	17,145	35,251	33,501
Insurance and claims	7,602	7,652	14,872	15,335
Communications and utilities	5,768	4,699	11,489	9,553
Depreciation and amortization	30,517	26,524	60,351	52,068
Purchased transportation	27,021	24,592	50,360	46,502
Building and office equipment rents	3,109	3,457	6,287	6,725
Miscellaneous expenses, net	17	2,665	1,614	5,294
Total operating expenses	492,731	464,864	965,203	913,465
Operating income	97,573	82,588	163,517	136,806
Non-operating expense (income):
Interest expense	2,403	2,685	4,803	5,904
Interest income	(42)	(33)	(56)	(85)
Other expense (income), net	334	875	408	529
Total non-operating expense	2,695	3,527	5,155	6,348
Income before income taxes	94,878	79,061	158,362	130,458
Provision for income taxes	36,623	31,229	59,554	51,531
Net income	$ 58,255	$ 47,832	$ 98,808	$ 78,927
Earnings per share:
Basic	$ 0.68	$ 0.56	$ 1.15	$ 0.92
Diluted	$ 0.68	$ 0.56	$ 1.15	$ 0.92
Weighted average shares outstanding:
Basic	86,164,917	86,164,986	86,164,917	86,164,986
Diluted	86,164,917	86,164,986	86,164,917	86,164,986