Long-Term Obligations (Narrative) (Details) (USD $)	0 Months Ended	9 Months Ended
	Oct. 31, 2007	Sep. 30, 2011	Dec. 31, 2010
Revolving credit agreements, maximum borrowing capacity		$ 450,000,000
Line of credit facility, initiation date		September 15, 2011
Line of credit facility, years		5
Line of credit facility, sub-facility		60,000,000
Standby letters of credit issued		33,800,000
Line of credit facility, maturity date		September 15, 2016
Borrowings outstanding under credit facility		80,000,000
Debt instrument, frequency of fee		quarterly
Line of credit facility fee, minimum range, percentage		0.175%
Line of credit facility fee, maximum range, percentage		0.275%
Default rate in addition to interest rate		2.00%
Convertible debt, interest rate		2.75%
Base conversion rate		30.1100
Base conversion price, per share		$ 33.21
2.75% Series A [Member]
Convertible senior debentures, issuance date	October 31, 2007
Convertible debt	275,000,000	275,000,000	275,000,000
Debentures principal amount for conversion calculation		1,000
Convertible debt, right to redeem, earliest date		November 1, 2012
Debt instrument, rate of return without conversion feature		7.00%
Debt instrument, expected useful life, years		5
Convertible debt, proceeds excess fair value of liability reclassified to equity	47,400,000
2.75% Series B [Member]
Convertible senior debentures, issuance date	October 31, 2007
Convertible debt	275,000,000	275,000,000	275,000,000
Debentures principal amount for conversion calculation		1,000
Convertible debt, right to redeem, earliest date		November 1, 2014
Debt instrument, rate of return without conversion feature		7.40%
Debt instrument, expected useful life, years		7
Convertible debt, proceeds excess fair value of liability reclassified to equity	$ 67,200,000