Condensed Statements Of Operations - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenue from operations	$ 925,020,000	$ 754,096,000
Operating expenses:
Salaries, wages and benefits	501,311,000	416,504,000
Operating supplies and expenses	114,061,000	90,987,000
General supplies and expenses	29,976,000	22,872,000
Operating taxes and licenses	26,788,000	24,022,000
Insurance and claims	11,099,000	8,790,000
Communications and utilities	7,046,000	7,433,000
Depreciation and amortization	53,481,000	50,287,000
Purchased transportation	21,740,000	17,997,000
Building and office equipment rents	1,875,000	2,114,000
Miscellaneous expenses, net	8,303,000	4,968,000
Total operating expenses	775,680,000	645,974,000
Operating income	149,340,000	108,122,000
Non-operating expense (income):
Interest expense	11,000	595,000
Interest income	(456,000)	(35,000)
Other expense (income), net	2,299,000	409,000
Total non-operating expense	1,854,000	969,000
Income before income taxes	147,486,000	107,153,000
Provision for income taxes	38,153,000	41,361,000
Net income	$ 109,333,000	$ 65,792,000
Earnings per share:
Basic	$ 1.33	$ 0.80
Diluted	1.33	0.80
Dividends declared per share	$ 0.13	$ 0.10
Weighted average shares outstanding:
Basic	82,253,470	82,349,024
Diluted	82,356,048	82,444,458