Condensed Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenue from operations	$ 1,058,233	$ 872,987	$ 3,016,751	$ 2,466,995
Operating expenses:
Salaries, wages and benefits	536,513	461,799	1,560,073	1,320,207
Operating supplies and expenses	126,024	95,543	365,004	275,110
General supplies and expenses	31,209	28,785	91,076	79,940
Operating taxes and licenses	27,952	24,547	82,905	73,530
Insurance and claims	12,069	10,700	34,510	28,804
Communications and utilities	8,215	6,490	22,700	20,945
Depreciation and amortization	58,086	51,934	167,802	152,670
Purchased transportation	25,373	22,739	73,157	61,596
Building and office equipment rents	1,533	2,018	5,055	6,114
Miscellaneous expenses, net	2,874	4,557	16,263	15,650
Total operating expenses	829,848	709,112	2,418,545	2,034,566
Operating income	228,385	163,875	598,206	432,429
Non-operating expense (income):
Interest expense	29	555	51	1,792
Interest income	(778)	(228)	(1,902)	(332)
Other (income) expense, net	(70)	(977)	1,895	(999)
Total non-operating (income) expense	(819)	(650)	44	461
Income before income taxes	229,204	164,525	598,162	431,968
Provision for income taxes	55,762	62,211	151,953	165,444
Net income	$ 173,442	$ 102,314	$ 446,209	$ 266,524
Earnings per share:
Basic	$ 2.12	$ 1.24	$ 5.44	$ 3.24
Diluted	2.12	1.24	5.43	3.23
Dividends declared per share	$ 0.13	$ 0.10	$ 0.39	$ 0.30
Weighted average shares outstanding:
Basic	81,885,262	82,286,295	82,067,519	82,317,244
Diluted	81,975,774	82,380,936	82,165,731	82,417,557