Debt (Schedule Of Debt) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Long-term debt	$ 1,245,014,000	$ 1,156,568,000
Notes issued with a discount	9,986,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term debt	265,000,000	177,966,000
Revolving Credit Facility maturity date	December 2012
Weighted average rate	1.00%	1.50%
Senior note interest payable dates	interest payable periodically at variable interest rates.
Senior Note 6.25% Due 2013 [Member]
Debt Instrument [Line Items]
Long-term debt	375,000,000	375,000,000
Senior Notes maturity date	March 2013
Interest rate stated percentage of Senior Notes	6.25%
Senior note interest payable dates	interest payable semi-annually in March and September at 6.25%
Senior Note 6.75% Due 2014 [Member]
Debt Instrument [Line Items]
Long-term debt	150,000,000	150,000,000
Senior Notes maturity date	January 2014
Interest rate stated percentage of Senior Notes	6.75%
Senior note interest payable dates	interest payable semi-annually in January and July at 6.75%
Senior Note 7.75% Due 2017 [Member]
Debt Instrument [Line Items]
Long-term debt	455,014,000	453,602,000
Senior Notes maturity date	June 2017
Interest rate stated percentage of Senior Notes	7.75%
Senior note interest payable dates	interest payable semi-annually in June and December at 7.75%
Notes issued with a discount	13,400,000
Notes, unamortized	$ 10,000,000	$ 11,400,000