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Long-Term Debt (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
 6 Months Ended 12 Months Ended 6 Months Ended 1 Months Ended 6 Months Ended 3 Months Ended
Jun. 30, 2012
Jun. 30, 2012
Line of Credit [Member]
Jun. 30, 2012
Subsequent Event [Member]
Dec. 31, 2011
2019 Notes [Member]
Dec. 31, 2011
2021 Notes [Member]
Jun. 30, 2012
Unsecured Senior Notes [Member]
Jul. 02, 2012
Unsecured Senior Notes [Member]
Subsequent Event [Member]
Jun. 30, 2012
Amended Credit Facility [Member]
Jun. 30, 2012
Maximum [Member]
Amended Credit Facility [Member]
Apr. 03, 2012
Maximum [Member]
Amended Credit Facility [Member]
Jun. 30, 2012
Maximum [Member]
Amended Credit Facility [Member]
EBITDAX [Member]
Jun. 30, 2011
Minimum [Member]
Amended Credit Facility [Member]
Current Ratio [Member]
Debt Disclosure [Line Items]                        
Senior unsecured debt principal amount       $ 400.0 $ 400.0   $ 400.0          
Senior unsecured notes stated interest rate       7.25% 6.50%   6.875%          
Senior unsecured notes payment frequency           Semi-annually Semi-annually          
Net proceeds from issuance of notes           783.4 392.0          
Debt instrument, maturity date       Feb. 01, 2019 Nov. 01, 2021   Jan. 15, 2023 Oct. 06, 2016        
Redemption percentage, maximum           35.00%            
Percentage of minimum outstanding principal amount           65.00%            
Event of default by payment default           10.0            
Event of default by final judgments           10.0            
Letters of credit borrowings   1,000             500 350    
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
Letter of credit unused borrowing base capacity               500        
Deferred financing costs     $ 25.2