Statements Of Operations (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenue from operations	$ 2,787,897	$ 2,337,648	$ 2,134,579
Operating expenses
Salaries, wages and benefits	1,381,277	1,170,773	1,066,551
Operating supplies and expenses	432,675	385,201	378,534
General supplies and expenses	83,165	69,765	58,908
Operating taxes and licenses	83,417	71,599	67,526
Insurance and claims	36,145	30,910	29,681
Communications and utilities	25,507	23,142	19,980
Depreciation and amortization	146,466	127,072	110,743
Purchased transportation	129,312	106,435	94,522
Building and office equipment rents	10,679	11,920	13,514
Miscellaneous expenses, net	17,947	2,393	9,366
Total operating expenses	2,346,590	1,999,210	1,849,325
Operating income	441,307	338,438	285,254
Non-operating expense (income)
Interest expense	6,610	9,620	11,541
Interest income	(108)	(147)	(113)
Other expense, net	2,291	279	728
Total non-operating expense	8,793	9,752	12,156
Income before income taxes	432,514	328,686	273,098
Provision for income taxes	165,000	122,573	103,646
Net income	$ 267,514	$ 206,113	$ 169,452
Earnings per share
Basic	$ 3.10	$ 2.39	$ 1.97
Diluted	$ 3.10	$ 2.39	$ 1.97
Weighted average shares outstanding
Basic	86,162,137	86,164,917	86,164,964
Diluted	86,162,137	86,164,917	86,164,964