Consolidated Balance Sheet (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Cash and cash equivalents	$ 498,792	$ 484,502
Trade accounts receivable, net	688,353	575,013
Other accounts receivable	50,130	51,063
Operating supplies, at lower of average cost or market	25,280	23,910
Prepaid expenses and other current assets	59,426	57,961
Deferred income taxes	23,598	15,332
Total current assets	1,345,579	1,207,781
Land	192,070	193,364
Buildings and leasehold improvements	855,274	856,038
Revenue equipment	1,864,453	1,857,737
Other equipment	350,554	353,205
Property, Plant and Equipment, Gross	3,262,351	3,260,344
Accumulated depreciation	(1,633,930)	(1,603,511)
Property, Plant and Equipment, Net	1,628,421	1,656,833
Deferred charges and other assets	32,808	32,200
Capitalized computer software, net	22,285	21,488
Employee benefits	15,688	15,018
Intangible assets, net	7,462	8,640
Goodwill	337,973	337,971
Total noncurrent assets	416,216	415,317
Total assets	3,390,216	3,279,931
Accounts payable	402,604	390,537
Accrued liabilities	247,769	229,078
Self-insurance accruals	115,534	105,063
Short-term borrowings	2,388	1,588
Current maturities of capital leases	27,435	19,685
Total current liabilities	795,730	745,951
Long-term debt	719,228	719,155
Long-term obligations under capital leases	9,527	16,185
Self-insurance accruals	141,989	142,307
Employee benefits	206,477	240,171
Other liabilities and deferred credits	38,827	39,524
Deferred income taxes	267,844	237,949
Total liabilities	2,179,622	2,141,242
Commitments and Contingencies
Common stock, $0.625 par value; authorized 100,000,000 shares; issued 65,269,344 and 64,592,756 shares, respectively	40,781	40,349
Additional paid in capital, common stock	677,278	653,487
Retained earnings	1,092,867	1,043,472
Cost of repurchased stock (7,755,426 and 7,669,889 shares, respectively)	(332,530)	(329,088)
Accumulated other comprehensive loss	(267,802)	(269,531)
Total stockholders' equity	1,210,594	1,138,689
Total liabilities and shareholders' equity	$ 3,390,216	$ 3,279,931