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Derivative Instruments and Hedging Activities - Before Tax Effect of Derivative Instruments in Statement of Operations (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives $ 4.6 $ 22.9
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion) (a) 12.8 36.8
Decrease/increase in oil revenue owing to effective hedging contracts 9.3 34.0
Decrease/increase in gas revenue owing to effective hedging contracts 3.5 2.8
Designated as Hedging Instrument | Cash Flow Hedging    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives 4.6 22.9
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion) (a) 12.8 36.8
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion) 0.1 0.9
Designated as Hedging Instrument | Cash Flow Hedging | Derivative Income (Expense), Net    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion) 0.1 0.9
Cash Flow Hedges    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives 4.6 22.9
Cash Flow Hedges | Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives 4.6 22.9
Reclassification out of Accumulated Other Comprehensive Income | Cash Flow Hedges | Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion) (a) $ 12.8 $ 36.8