Condensed Statements Of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Revenue from operations	$ 990,782	$ 925,020
Operating expenses:
Salaries, wages and benefits	522,344	501,311
Operating supplies and expenses	121,357	115,936
General supplies and expenses	31,560	29,976
Operating taxes and licenses	29,071	26,788
Insurance and claims	11,172	11,099
Communications and utilities	7,839	7,046
Depreciation and amortization	63,073	53,481
Purchased transportation	20,687	21,740
Miscellaneous expenses, net	5,253	8,303
Total operating expenses	812,356	775,680
Operating income	178,426	149,340
Non-operating (income) expense:
Interest expense	122	11
Interest income	(1,483)	(456)
Other (income) expense, net	(600)	2,299
Total non-operating (income) expense	(1,961)	1,854
Income before income taxes	180,387	147,486
Provision for income taxes	47,064	38,153
Net income	$ 133,323	$ 109,333
Earnings per share:
Basic	$ 1.65	$ 1.33
Diluted	$ 1.64	$ 1.33
Weighted average shares outstanding:
Basic	81,033,137	82,253,470
Diluted	81,143,554	82,356,048
Dividends declared per share	$ 0.17	$ 0.13