Consolidated Balance Sheet (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Cash and cash equivalents	$ 405,732	$ 484,502
Trade accounts receivable, net	657,932	575,013
Other accounts receivable	51,226	51,063
Operating supplies, at lower of average cost or market	24,780	23,910
Prepaid expenses and other current assets	75,227	57,961
Deferred income taxes	15,411	15,332
Total current assets	1,230,308	1,207,781
Land	193,292	193,364
Buildings and leasehold improvements	857,089	856,038
Revenue equipment	1,855,435	1,857,737
Other equipment	347,290	353,205
Property, Plant and Equipment, Gross	3,253,106	3,260,344
Accumulated depreciation	(1,615,093)	(1,603,511)
Property, Plant and Equipment, Net	1,638,013	1,656,833
Deferred charges and other assets	32,977	32,200
Capitalized computer software, net	21,544	21,488
Employee benefits	15,342	15,018
Intangible assets, net	8,051	8,640
Goodwill	337,898	337,971
Total noncurrent assets	415,812	415,317
Total assets	3,284,133	3,279,931
Accounts payable	392,642	390,537
Accrued liabilities	230,456	229,078
Self-insurance accruals	117,429	105,063
Short-term borrowings	1,876	1,588
Current maturities of capital leases	19,357	19,685
Total current liabilities	761,760	745,951
Long-term debt	719,191	719,155
Long-term obligations under capital leases	17,457	16,185
Self-insurance accruals	146,316	142,307
Employee benefits	211,383	240,171
Other liabilities and deferred credits	37,294	39,524
Deferred income taxes	241,616	237,949
Total liabilities	2,135,017	2,141,242
Commitments and Contingencies
Common stock, $0.625 par value; authorized 100,000,000 shares; issued 64,781,259 and 64,592,756 shares, respectively	40,467	40,349
Additional paid in capital, common stock	657,630	653,487
Retained earnings	1,050,662	1,043,472
Cost of repurchased stock (7,727,595 and 7,669,889 shares, respectively)	(331,288)	(329,088)
Accumulated other comprehensive loss	(268,355)	(269,531)
Total stockholders' equity	1,149,116	1,138,689
Total liabilities and shareholders' equity	$ 3,284,133	$ 3,279,931