Long-Term Obligations (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended
	Oct. 31, 2007	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
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
Borrowings outstanding under credit facility		100,000,000
Standby letter of credit outstanding		33,800,000
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		30.5977
Base conversion price, per share		$ 32.68
Series A [Member]
Debt Instrument [Line Items]
Convertible senior debentures, issuance date	October 31, 2007,
Convertible debt		265,941,000	255,727,000
Convertible debt, principal amount	275,000,000	275,000,000	275,000,000
Exercise of option allotment amount	25,000,000
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
Interest expense accretion period, in years		5
Remaining period that discount will be amortized, months		10
Convertible debt, maturity date		2037
Series B [Member]
Debt Instrument [Line Items]
Convertible senior debentures, issuance date	October 31, 2007
Convertible debt		243,668,000	234,047,000
Convertible debt, principal amount	275,000,000	275,000,000	275,000,000
Exercise of option allotment amount	25,000,000
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
Interest expense accretion period, in years		7
Remaining period that discount will be amortized, months		34
Convertible debt, maturity date		2037