Statements Of Consolidated Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Statement of Cash Flows [Abstract]
Cash and Cash Equivalents, Beginning of Period	$ 438,010	$ 421,420
Operating Activities
Net Income	67,446	36,344
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Depreciation and amortization, net of accretion	104,785	100,364
Non-cash compensation and employee benefits	17,250	12,207
Increase in deferred income taxes	41,139	26,224
Provision for uncollectible accounts	2,426	3,159
Gain from sales of property and equipment, net	(6,446)	(1,632)
Changes in assets and liabilities:
Receivables	(90,218)	(96,076)
Prepaid expenses	596	2,055
Accounts payable	51,792	63,386
Accrued variable compensation	(14,251)	(1,506)
Accrued liabilities, excluding accrued variable compensation and employee benefits	35,496	25,000
Self-insurance accruals	(10,947)	(14,858)
Accrued income taxes	(7,676)	29,942
Employee benefits	(20,337)	1,389
Deferred charges and credits	(4,611)	1,699
Other	(4,543)	(1,751)
Net Cash Provided by Operating Activities	161,901	185,946
Investing Activities
Capital expenditures	(148,669)	(123,468)
Software expenditures	(3,076)	(3,854)
Proceeds from sales of property and equipment	15,721	4,525
Purchases of marketable securities	(5,000)	0
Proceeds from sales of marketable securities	13,330	300
Net Cash Used in Investing Activities	(127,694)	(122,497)
Financing Activities
Repayment of capital leases	(9,657)	(9,272)
Net proceeds from (repayments of) short-term borrowings	(312)	245
Proceeds from exercise of stock options	2,396	4,087
Excess tax benefit from share-based compensation	1,510	544
Payments of common dividends	(11,165)	(11,060)
Net Cash Used in Financing Activities	(17,228)	(15,456)
Increase in Cash and Cash Equivalents	16,979	47,993
Cash and Cash Equivalents, End of Period	454,989	469,413
Supplemental Disclosure
Cash paid (refunded) for income taxes, net	6,160	(28,881)
Cash paid for interest, net of amounts capitalized	27,012	27,421
Non-cash Investing and Financing Activities
Revenue equipment acquired through partial non-monetary exchanges	13,658	15,366
Repurchased common stock issued under defined contribution plan	$ 0	$ 17,307