Statements of Consolidated Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Revenue	$ 1,427,314	$ 1,492,349	$ 2,799,745	$ 2,861,192
Salaries, wages and employee benefits	584,031	561,073	1,148,338	1,098,325
Purchased transportation	328,298	368,658	660,232	701,643
Other operating expenses	176,885	157,510	343,990	319,746
Fuel and fuel-related taxes	88,522	130,802	175,425	267,504
Depreciation and amortization	59,725	60,848	119,688	120,459
Purchased labor	35,127	42,334	75,637	84,554
Rents and leases	36,939	34,399	72,866	68,358
Maintenance	33,781	34,025	67,636	64,841
Operating Expenses	1,343,308	1,389,649	2,663,812	2,725,430
Operating Income	84,006	102,700	135,933	135,762
Investment income	144	175	309	336
Interest expense	(13,221)	(13,403)	(26,498)	(26,709)
Miscellaneous, net	524	1,296	(1,896)	601
Other Income (Expense)	(12,553)	(11,932)	(28,085)	(25,772)
Income before income tax provision	71,453	90,768	107,848	109,990
Income tax provision	27,418	37,101	42,021	43,430
Net Income	$ 44,035	$ 53,667	$ 65,827	$ 66,560
Weighted average common shares outstanding, basic	57,419,971	57,128,379	57,526,585	57,043,378
Weighted average common shares outstanding, diluted	57,805,354	57,694,691	58,013,169	57,577,373
Earnings per common share, basic	$ 0.77	$ 0.94	$ 1.14	$ 1.17
Earnings per common share, diluted	0.76	0.93	1.13	1.16
Cash Dividends Declared per Common Share	$ 0.30	$ 0.20	$ 0.45	$ 0.30