Statement of Income (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Consolidated Income Statement [Abstract]
Revenue	$ 1,492,349	$ 1,381,370	$ 2,861,192	$ 2,717,534
Salaries, wages and employee benefits	561,073	539,556	1,098,325	1,057,397
Purchased transportation	368,658	329,021	701,643	677,544
Other operating expenses	157,510	147,117	319,746	304,814
Fuel and fuel-related taxes	130,802	135,547	267,504	272,959
Depreciation and amortization	60,848	57,235	120,459	113,163
Purchased labor	42,334	34,045	84,554	62,027
Rents and leases	34,399	31,136	68,358	60,986
Maintenance	34,025	31,414	64,841	60,746
Operating Expenses	1,389,649	1,305,071	2,725,430	2,609,636
Operating Income	102,700	76,299	135,762	107,898
Investment income	175	159	336	323
Interest expense	(13,403)	(13,659)	(26,709)	(27,164)
Miscellaneous, net	1,296	50	601	(1,433)
Other Income (Expense)	(11,932)	(13,450)	(25,772)	(28,274)
Income before income tax provision	90,768	62,849	109,990	79,624
Income tax provision	37,101	19,952	43,430	22,722
Net income	$ 53,667	$ 42,897	$ 66,560	$ 56,902
Weighted average common shares outstanding, basic	57,128,379	56,354,017	57,043,378	56,226,038
Weighted average common shares outstanding, diluted	57,694,691	56,960,738	57,577,373	56,860,095
Earnings per common share, basic	$ 0.94	$ 0.76	$ 1.17	$ 1.01
Earnings per common share, diluted	$ 0.93	$ 0.75	$ 1.16	$ 1.00
Cash dividends declared per common share	$ 0.10	$ 0.10	$ 0.20	$ 0.20