Condensed Statements Of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income Statement [Abstract]
Revenue from operations	$ 707,733	$ 696,245
Operating expenses:
Salaries, wages and benefits	400,869	368,442
Operating supplies and expenses	75,372	88,049
General supplies and expenses	21,142	21,292
Operating taxes and licenses	23,188	22,274
Insurance and claims	10,244	10,042
Communications and utilities	7,005	6,775
Depreciation and amortization	44,772	38,788
Purchased transportation	18,496	30,148
Building and office equipment rents	2,273	2,278
Miscellaneous expenses, net	4,824	4,592
Total operating expenses	608,185	592,680
Operating income	99,548	103,565
Non-operating expense (income):
Interest expense	1,183	1,569
Interest income	(16)	(71)
Other expense (income), net	516	237
Total non-operating expense	1,683	1,735
Income before income taxes	97,865	101,830
Provision for income taxes	37,580	39,306
Net income	$ 60,285	$ 62,524
Earnings per share:
Basic	$ 0.72	$ 0.73
Diluted	$ 0.72	$ 0.73
Weighted average shares outstanding:
Basic	83,983,028	85,970,829
Diluted	83,983,028	85,970,829