Debt (Schedule of Debt) (Details) (USD $)	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Long-term debt	$ 1,104,294,000	$ 1,156,568,000
Senior note interest payable dates	interest payable periodically at variable interest rates
Revolving Credit Facility [Member]
Long-term debt	125,000,000	177,966,000
Revolving Credit Facility maturity date	December 2012
Weighted average rate	0.90%	1.50%
Revolving Credit Facility interest payable terms	interest payable periodically at variable interest rates.
Senior Note 6.25% Due 2013 [Member]
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]
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]
Long-term debt	454,294,000	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	$ 10,700,000	$ 11,400,000