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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 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives $ (8.6) $ (18.8) $ (4.0) $ 4.1
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion) (a) 8.9 30.4 21.7 67.2
Decrease/increase in oil revenue owing to effective hedging contracts 5.1 26.4 14.4 60.4
Decrease/increase in gas revenue owing to effective hedging contracts 3.8 4.0 7.3 6.8
Designated as Hedging Instrument | Cash Flow Hedging        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives (8.6) (18.8) (4.0) 4.1
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion) (a) 8.9 30.4 21.7 67.2
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion) (0.6) (0.4) (0.5) 0.5
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)     (0.5) 0.5
Cash Flow Hedges        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives (8.6) (18.8) (4.0) 4.1
Cash Flow Hedges | Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives (8.6) (18.8) (4.0) 4.1
Cash Flow Hedges | Designated as Hedging Instrument | Derivative Income (Expense), Net        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion) (0.6) (0.4)    
Reclassification out of Accumulated Other Comprehensive Income | Cash Flow Hedges        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion) (a) 8.9   21.7  
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) $ 8.9 $ 30.4 $ 21.7 $ 67.2