Long-Term Debt (Details) (USD $)	6 Months Ended				3 Months Ended			3 Months Ended			3 Months Ended					1 Months Ended		1 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Revolving credit facility	Jun. 15, 2011 Revolving credit facility	Mar. 31, 2011 Senior Notes	Jun. 30, 2011 Senior Notes	Dec. 31, 2010 Senior Notes	Mar. 31, 2011 Senior Notes, 8.5% interest, issued July 2006 and due July 2016	Jun. 30, 2011 Senior Notes, 8.5% interest, issued July 2006 and due July 2016	Dec. 31, 2010 Senior Notes, 8.5% interest, issued July 2006 and due July 2016	Mar. 31, 2011 Senior Notes, 8.75% interest, issued April and May 2008 and due April 2018	Jun. 30, 2011 Senior Notes, 8.75% interest, issued April and May 2008 and due April 2018	Dec. 31, 2010 Senior Notes, 8.75% interest, issued April and May 2008 and due April 2018	Jun. 30, 2011 Senior Notes, 6.75% interest, issued November 2010 and due November 2020	Dec. 31, 2010 Senior Notes, 6.75% interest, issued November 2010 and due November 2020	Mar. 31, 2011 Senior Notes, 6.5% interest, issued February and March 2011 and due August 2021	Jun. 30, 2011 Senior Notes, 6.5% interest, issued February and March 2011 and due August 2021	Mar. 31, 2011 Senior Notes, 6.5% interest, issued February 2011 and due August 2021	Mar. 31, 2011 Senior Notes, 6.5% interest, issued March 2011 and due August 2021
Long-Term Debt
Long-term debt, net of discounts	$ 1,582,102,000	$ 1,273,434,000	$ 246,500,000						$ 2,784,000	$ 274,358,000		$ 333,786,000	$ 499,076,000	$ 500,000,000	$ 500,000,000		$ 499,032,000
Debt instrument, stated interest rate percentage (expressed as a decimal)			4.22%						8.50%			8.75%		6.75%			6.50%
Long-term debt, discounts	1,550,000	1,566,000							6,000	642,000		576,000	924,000				968,000
Estimate aggregate fair value of debt						1,366,200,000	1,333,900,000
Credit facility maximum borrowing capacity				745,000,000
Uncommitted accordion feature				155,000,000
Letters of credit outstanding amount			27,300,000
Credit facility remaining borrowing capacity			471,200,000
Aggregate principal amount of public offering																		200,000,000	300,000,000
Issue price as percentage of par value (as a percent)																		99.50%
Aggregate net proceeds from issuance of debt																492,000,000
Aggregate principal amount of debt repurchased								272,200,000			165,600,000
Pre-tax loss on redemption of debt	(43,328,000)				43,300,000
Write off of the unamortized discount and deferred finance costs on extinguishment of debt					3,800,000
Tender premiums and third-party expenses					$ 39,500,000