Statement of Income (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]
Revenue	$ 1,504,150	$ 1,398,021	$ 4,365,342	$ 4,115,555
Salaries, wages and employee benefits	570,216	541,373	1,668,541	1,598,770
Purchased transportation	379,141	327,691	1,080,784	1,005,235
Other operating expenses	164,015	162,736	483,761	467,550
Fuel and fuel-related taxes	122,479	133,093	389,983	406,052
Depreciation and amortization	60,848	58,911	181,307	172,074
Purchased labor	46,219	38,750	130,773	100,777
Rents and leases	34,004	33,778	102,362	94,764
Maintenance	35,853	34,014	100,694	94,760
Operating Expenses	1,412,775	1,330,346	4,138,205	3,939,982
Operating Income	91,375	67,675	227,137	175,573
Investment income	185	150	521	473
Interest expense	(13,373)	(13,395)	(40,082)	(40,559)
Miscellaneous, net	(797)	(1,052)	(196)	(2,485)
Other Income (Expense)	(13,985)	(14,297)	(39,757)	(42,571)
Income Before Income Tax Provision	77,390	53,378	187,380	133,002
Income tax provision	31,807	22,821	75,237	45,543
Net Income	$ 45,583	$ 30,557	$ 112,143	$ 87,459
Weighted-average common shares outstanding, basic	57,692,508	56,714,423	57,262,132	56,390,621
Weighted-average common shares outstanding, diluted	58,253,563	57,362,834	57,832,133	57,065,146
Earnings per common share, basic	$ 0.79	$ 0.54	$ 1.96	$ 1.55
Earnings per common share, diluted	$ 0.78	$ 0.53	$ 1.94	$ 1.53
Cash dividends declared per common share	$ 0.20	$ 0.10	$ 0.50	$ 0.40