Long-Term Obligations (Narrative) (Details) (USD $)	0 Months Ended	3 Months Ended	12 Months Ended
	Oct. 31, 2007	Mar. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Revolving credit agreement, maximum borrowing capacity		$ 450,000,000
Line of credit facility, initiation date		September 15, 2011
Revolving credit facility, years		5
Letter of credit sub-facility		60,000,000
Borrowings outstanding under credit facility		170,000,000
Standby letters of credit issued		36,800,000
Debt instrument, frequency of fee		quarterly
Letter of credit facility fee, minimum range, percentage		0.175%
Letter of credit facility fee, maximum range, percentage		0.275%
Interest rate per annum added to default rate		2.00%
Convertible debt, interest rate		2.75%
Base conversion rate		30.5977
Base conversion price, per share		$ 32.68
2.75% Series A [Member]
Debt Instrument [Line Items]
Convertible senior debentures, issuance date	October 31, 2007
Convertible debt, principal amount	275,000,000	275,000,000	275,000,000
Exercise of option over-allotment amount	25,000,000
Debentures principal amount for conversion calculation		1,000
Convertible debt, right to redeem, earliest date		November 1, 2012
Number of years within which holders to put securities for cash		1	1
Debt instrument, rate of return without conversion feature		7.00%
Debt instrument expected lives, 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		7
Convertible debt, maturity date		2037
2.75% Series B [Member]
Debt Instrument [Line Items]
Convertible senior debentures, issuance date	October 31, 2007
Convertible debt, principal amount	275,000,000	275,000,000	275,000,000
Exercise of option over-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 lives, 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		31
Convertible debt, maturity date		2037
Series Debentures [Member]
Debt Instrument [Line Items]
Debentures principal amount for conversion calculation		$ 1,000