Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Cash and cash equivalents	$ 435,080	$ 432,759
Marketable securities	8,285	8,285
Trade accounts receivable, net	673,082	649,086
Other accounts receivable	65,966	70,305
Operating supplies, at lower of average cost or market	22,938	23,664
Prepaid expenses and other current assets	63,635	63,344
Deferred income taxes	13,567	13,957
Total Current Assets	1,282,553	1,261,400
Land	192,471	192,490
Buildings and leasehold improvements	860,558	856,037
Revenue equipment	1,931,121	1,902,358
Other equipment	360,935	362,341
Property, plant and equipment, gross	3,345,085	3,313,226
Accumulated depreciation	(1,697,517)	(1,659,015)
Net Property, Plant and Equipment	1,647,568	1,654,211
Deferred charges and other assets	30,208	31,826
Capitalized software, net	30,155	26,208
Employee benefits	18,675	18,110
Intangible assets, net	5,106	6,284
Goodwill	337,456	337,579
Total Noncurrent Assets	421,600	420,007
Total Assets	3,351,721	3,335,618
Accounts payable	394,364	349,995
Accrued liabilities	260,688	257,943
Self-insurance accruals	117,737	117,783
Short-term borrowings	2,045	1,736
Current maturities of capital leases	3,224	14,663
Total Current Liabilities	778,058	742,120
Long-term debt	719,381	719,303
Long-term obligations under capital leases	8,811	10,587
Self-insurance accruals	143,122	151,257
Employee benefits	216,963	239,368
Other liabilities and deferred credits	32,382	34,356
Deferred income taxes	250,110	242,789
Total Liabilities	$ 2,148,827	$ 2,139,780
Commitments and Contingencies (Note 9)
Common stock ($0.625 par value; authorized 100,000,000 shares; issued 66,303,416 and 65,782,041 shares, respectively)	$ 41,471	$ 41,101
Additional paid in capital, common stock	716,953	706,756
Retained earnings	1,191,797	1,151,791
Cost of repurchased stock (8,673,724 and 8,112,141 shares, respectively)	(393,919)	(349,401)
Accumulated other comprehensive loss	(353,408)	(354,409)
Total Stockholders' Equity	1,202,894	1,195,838
Total Liabilities and Shareholders' Equity	$ 3,351,721	$ 3,335,618