Statements Of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Revenue from operations	$ 2,972,442	$ 2,787,897	$ 2,337,648
Operating expenses
Salaries, wages and benefits	1,569,791	1,381,277	1,170,773
Operating supplies and expenses	353,889	432,675	385,201
General supplies and expenses	89,308	83,165	69,765
Operating taxes and licenses	93,292	83,417	71,599
Insurance and claims	37,368	36,145	30,910
Communications and utilities	26,913	25,507	23,142
Depreciation and amortization	165,343	146,466	127,072
Purchased transportation	116,300	129,312	106,435
Building and office equipment rents	9,620	10,679	11,920
Miscellaneous expenses, net	12,378	17,947	2,393
Total operating expenses	2,474,202	2,346,590	1,999,210
Operating income	498,240	441,307	338,438
Non-operating expense (income)
Interest expense	5,210	6,610	9,620
Interest income	(209)	(108)	(147)
Other expense, net	3,222	2,291	279
Total non-operating expense	8,223	8,793	9,752
Income before income taxes	490,017	432,514	328,686
Provision for income taxes	185,327	165,000	122,573
Net income	$ 304,690	$ 267,514	$ 206,113
Earnings per share
Basic	$ 3.57	$ 3.10	$ 2.39
Diluted	$ 3.57	$ 3.10	$ 2.39
Weighted average shares outstanding
Basic	85,378,480	86,162,137	86,164,917
Diluted	85,378,480	86,162,137	86,164,917