Consolidated Balance Sheet (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Cash and cash equivalents	$ 456,724	$ 484,502
Marketable securities	3,285	0
Trade accounts receivable, net	694,533	575,013
Other accounts receivable	50,099	51,063
Operating supplies, at lower of average cost or market	24,369	23,910
Prepaid expenses and other current assets	48,396	57,961
Deferred income taxes	6,770	15,332
Total Current Assets	1,284,176	1,207,781
Land	188,909	193,364
Buildings and leasehold improvements	856,780	856,038
Revenue equipment	1,899,113	1,857,737
Other equipment	356,776	353,205
Property, Plant and Equipment, Gross	3,301,578	3,260,344
Accumulated depreciation	(1,647,699)	(1,603,511)
Property, Plant and Equipment, Net	1,653,879	1,656,833
Deferred charges and other assets	35,888	32,200
Capitalized software, net	23,048	21,488
Employee benefits	16,024	15,018
Intangible assets, net	6,873	8,640
Goodwill	337,955	337,971
Total Noncurrent Assets	419,788	415,317
Total Assets	3,357,843	3,279,931
Accounts payable	405,209	390,537
Accrued liabilities	270,325	229,078
Federal and other income taxes	7,937	0
Self-insurance accruals	116,248	105,063
Short-term borrowings	1,489	1,588
Current maturities of capital leases	25,256	19,685
Total Current Liabilities	826,464	745,951
Long-term debt	719,265	719,155
Long-term obligations under capital leases	10,791	16,185
Self-insurance accruals	146,605	142,307
Employee benefits	93,970	240,171
Other liabilities and deferred credits	34,672	39,524
Deferred income taxes	262,045	237,949
Total Liabilities	2,093,812	2,141,242
Commitments and Contingencies
Common stock, $0.625 par value; authorized 100,000,000 shares; issued 65,767,105 and 64,592,756 shares, respectively	41,092	40,349
Additional paid in capital, common stock	700,717	653,487
Retained earnings	1,126,884	1,043,472
Cost of repurchased stock (7,847,141 and 7,669,889 shares, respectively)	(337,228)	(329,088)
Accumulated other comprehensive loss	(267,434)	(269,531)
Total Shareholders' Equity	1,264,031	1,138,689
Total Liabilities and Shareholders' Equity	$ 3,357,843	$ 3,279,931