Statements Of Operations (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Statement [Abstract]
Revenue from operations	$ 2,110,483	$ 1,882,541	$ 1,480,998
Operating expenses
Salaries, wages and benefits	1,066,551	956,079	808,819
Operating supplies and expenses	378,534	355,186	244,291
General supplies and expenses	58,908	49,900	41,580
Operating taxes and licenses	67,526	63,284	55,420
Insurance and claims	29,681	27,693	25,329
Communications and utilities	19,980	18,104	15,218
Depreciation and amortization	110,743	90,820	80,362
Purchased transportation	70,426	63,257	50,489
Building and office equipment rents	13,514	13,689	15,244
Miscellaneous expenses, net	9,366	10,457	6,507
Total operating expenses	1,825,229	1,648,469	1,343,259
Operating income	285,254	234,072	137,739
Non-operating expense (income)
Interest expense	11,541	14,067	12,613
Interest income	(113)	(180)	(148)
Other expense, net	728	101	848
Total non-operating expense	12,156	13,988	13,313
Income before income taxes	273,098	220,084	124,426
Provision for income taxes	103,646	80,614	48,775
Net income	$ 169,452	$ 139,470	$ 75,651
Earnings per share
Basic	$ 1.97	$ 1.63	$ 0.90
Diluted	$ 1.97	$ 1.63	$ 0.90
Weighted average shares outstanding
Basic	86,164,964	85,719,728	83,890,483
Diluted	86,164,964	85,719,728	83,890,483