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Long-Term Debt (Narrative) (Details) (USD $)
 3 Months Ended 12 Months Ended 0 Months Ended 3 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
2019 Notes [Member]
Dec. 31, 2011
2021 Notes [Member]
Oct. 06, 2011
Amended Credit Facility [Member]
Mar. 31, 2012
Amended Credit Facility [Member]
Apr. 03, 2012
Amended Credit Facility [Member]
Feb. 02, 2011
Amended Credit Facility [Member]
Mar. 31, 2012
Unsecured Senior Notes [Member]
Dec. 31, 2011
Unsecured Senior Notes [Member]
Jun. 16, 2011
Line Of Credit [Member]
Oct. 06, 2011
Line Of Credit [Member]
Maximum [Member]
Mar. 31, 2012
EBITDAX [Member]
Maximum [Member]
Amended Credit Facility [Member]
Mar. 31, 2012
Current Ratio [Member]
Minimum [Member]
Amended Credit Facility [Member]
Debt Instrument [Line Items]                          
Percentage of the reserve value of assets as determined by reserve reports 80.00%                        
Commitment fee of borrowing base capacity not utilized 0.375%                        
Maximum debt to EBITDAX ratio                       4.0  
Minimum current ratio                         1.0
Letters of credit maximum borrowings       $ 350,000,000   $ 500,000,000 $ 137,500,000     $ 600,000,000 $ 1,000,000,000    
Letter of credit unused borrowing base capacity         350,000,000                
Senior unsecured debt principal amount   400,000,000 400,000,000                    
Senior unsecured notes stated interest rate   7.25% 6.50%                    
Debt instrument, maturity date   Feb. 01, 2019 Nov. 01, 2021 Feb. 26, 2015 Oct. 06, 2016                
Senior unsecured notes payment frequency               semi-annually          
Net proceeds from issuance of notes                 783,400,000        
Redemption percentage, maximum               35.00%          
Percentage of minimum outstanding principal amount               65.00%          
Event of default by payment default 10,000,000                        
Event of default by final judgments 10,000,000                        
Deferred financing costs $ 17,800,000