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Subsequent Events (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
6 Months Ended 1 Months Ended
Jun. 30, 2012
Unsecured Senior Notes [Member]
Jul. 02, 2012
Subsequent Event [Member]
Unsecured Senior Notes [Member]
Jul. 31, 2012
Subsequent Event [Member]
2012 Two-Way And Three-Way Collars [Member]
bbl
Jul. 31, 2012
Subsequent Event [Member]
2013 Two-Way And Three-Way Collars [Member]
bbl
Jul. 31, 2012
Subsequent Event [Member]
2014 Two-Way And Three-Way Collars [Member]
bbl
Subsequent Event [Line Items]          
Senior unsecured debt principal amount   $ 400.0      
Senior unsecured notes stated interest rate   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   Jan. 15, 2023      
Total notional amount, in barrels     183,000 1,215,500 108,500