Statements Of Consolidated Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Statement of Cash Flows [Abstract]
Cash and Cash Equivalents, Beginning of Period	$ 429,784	$ 438,010
Operating Activities
Net Income	56,902	67,446
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Depreciation and amortization, net of accretion	112,774	104,785
Non-cash compensation and employee benefits	18,275	17,250
Increase in deferred income taxes	26,911	41,139
Provision for uncollectible accounts	5,334	2,426
Gain from sales of property and equipment, net	(7,062)	(6,446)
Changes in assets and liabilities:
Receivables	(37,987)	(90,218)
Prepaid expenses	4,797	596
Accounts payable	16,793	51,792
Accrued variable compensation	(33,902)	(14,251)
Accrued liabilities, excluding accrued variable compensation and employee benefits	29,570	35,496
Self-insurance accruals	(3,859)	(10,947)
Accrued income taxes	(8,358)	(7,676)
Employee benefits	(48,286)	(20,337)
Other	3,880	(9,154)
Net Cash Provided by Operating Activities	135,782	161,901
Investing Activities
Capital expenditures	(129,273)	(148,669)
Software expenditures	(3,246)	(3,076)
Proceeds from sales of property and equipment	9,965	15,721
Purchases of marketable securities	0	(5,000)
Proceeds from sales of marketable securities	3,200	13,330
Net Cash Used in Investing Activities	(119,354)	(127,694)
Financing Activities
Repayment of capital leases	(6,373)	(9,657)
Repayments of short-term borrowings	(4,698)	(312)
Payment of debt issuance costs	(543)	0
Proceeds from exercise of stock options	8,987	2,396
Excess tax benefit from share-based compensation	1,057	1,510
Payments of common dividends	(11,255)	(11,165)
Net Cash Used in Financing Activities	(12,825)	(17,228)
Increase in Cash and Cash Equivalents	3,603	16,979
Cash and Cash Equivalents, End of Period	433,387	454,989
Supplemental Disclosure
Cash paid for income taxes, net	3,136	6,160
Cash paid for interest	26,516	27,012
Non-cash Investing and Financing Activities
Revenue equipment acquired through partial non-monetary exchanges	$ 16,872	$ 13,658