Condensed Statements Of Operations - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Revenue from operations	$ 839,912,000	$ 755,435,000	$ 1,594,008,000	$ 1,463,168,000
Operating expenses:
Salaries, wages and benefits	441,904,000	408,424,000	858,408,000	809,293,000
Operating supplies and expenses	88,580,000	80,335,000	179,567,000	155,707,000
General supplies and expenses	28,283,000	22,778,000	51,155,000	43,920,000
Operating taxes and licenses	24,961,000	23,466,000	48,983,000	46,654,000
Insurance and claims	9,314,000	9,363,000	18,104,000	19,607,000
Communications and utilities	7,022,000	7,327,000	14,455,000	14,332,000
Depreciation and amortization	50,449,000	46,480,000	100,736,000	91,252,000
Purchased transportation	20,860,000	18,176,000	38,857,000	36,672,000
Building and office equipment rents	1,982,000	2,164,000	4,096,000	4,437,000
Miscellaneous expenses, net	6,125,000	3,486,000	11,093,000	8,310,000
Total operating expenses	679,480,000	621,999,000	1,325,454,000	1,230,184,000
Operating income	160,432,000	133,436,000	268,554,000	232,984,000
Non-operating expense (income):
Interest expense	642,000	1,064,000	1,237,000	2,247,000
Interest income	(69,000)	(12,000)	(104,000)	(28,000)
Other expense (income), net	(431,000)	260,000	(22,000)	776,000
Total non-operating expense	142,000	1,312,000	1,111,000	2,995,000
Income before income taxes	160,290,000	132,124,000	267,443,000	229,989,000
Provision for income taxes	61,872,000	50,736,000	103,233,000	88,316,000
Net income	$ 98,418,000	$ 81,388,000	$ 164,210,000	$ 141,673,000
Earnings per share:
Basic	$ 1.20	$ 0.98	$ 1.99	$ 1.69
Diluted	1.19	0.98	1.99	1.69
Dividends declared per share	$ 0.10	$ 0.00	$ 0.20	$ 0.00
Weighted average shares outstanding:
Basic	82,318,623	83,354,013	82,333,739	83,668,521
Diluted	82,427,975	83,381,429	82,436,170	83,682,228