Long-Term Debt (Narrative) (Details) (USD $)	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Maturity duration (in years)	10
Maturity date	August 1, 2017
Conversion rate	28.1821
Principal amount of senior convertible debentures	$ 1,000
Conversion price per share	$ 35.48
Debentures principal amount	325,000,000	325,000,000
Line of credit facility, expiration date	October 18, 2012
Borrowing capacity provided	150,000,000
Credit facility, amount outstanding	0
Letters of credit outstanding	14,900,000
Maximum borrowing capacity	134,100,000
Aggregate principal amount	5,000,000
Credit availability	119,200,000
Short-Term Credit Facility [Member]
Debt Instrument [Line Items]
Borrowing capacity provided	6,000,000
Letters of credit outstanding	100,000
Credit availability	$ 5,900,000