Statements Of Consolidated Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Statement of Cash Flows [Abstract]
Cash and Cash Equivalents, Beginning of Period	$ 429,784	$ 438,010
Operating Activities
Net Income	87,459	92,750
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Depreciation and amortization, net of accretion	171,426	159,936
Non-cash compensation and employee benefits	27,797	25,299
Increase in deferred income taxes	46,784	57,102
Provision for uncollectible accounts	5,045	3,267
Gain from sales of property and equipment, net	(6,169)	(8,015)
Changes in assets and liabilities:
Receivables	(34,563)	(54,586)
Prepaid expenses	14,334	9,805
Accounts payable	21,044	8,716
Accrued variable compensation	(22,896)	(4,742)
Accrued liabilities, excluding accrued variable compensation and employee benefits	29,675	25,153
Self-insurance accruals	758	(10,042)
Accrued income taxes	(7,344)	(8,949)
Employee benefits	(78,540)	(55,865)
Other	4,853	(10,126)
Net Cash Provided by Operating Activities	259,663	229,703
Investing Activities
Capital expenditures	(212,549)	(213,217)
Software expenditures	(4,716)	(5,743)
Proceeds from sales of property and equipment	12,239	19,385
Purchases of marketable securities	0	(8,200)
Proceeds from sales of marketable securities	3,200	23,613
Net Cash Used in Investing Activities	(201,826)	(184,162)
Financing Activities
Payment of capital leases	(9,598)	(25,868)
Repayments of short-term borrowings	(4,673)	(3,891)
Payment of debt issuance costs	(543)	0
Proceeds from exercise of stock options	19,757	3,357
Excess tax benefit from share-based compensation	2,486	1,632
Payments of common dividends	(16,930)	(16,760)
Net Cash Used in Financing Activities	(9,501)	(41,530)
Increase in Cash and Cash Equivalents	48,336	4,011
Cash and Cash Equivalents, End of Period	478,120	442,021
Supplemental Disclosure
Cash paid for income taxes, net	3,692	7,260
Cash paid for interest	42,349	43,105
Non-cash Investing and Financing Activities
Revenue equipment acquired through partial non-monetary exchanges	23,683	23,974
Revenue equipment acquired through capital lease	$ 1,596	$ 0