Derivative Instruments and Hedging Activities - Before Tax Effect of Derivative Instruments in Statement of Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Decrease/increase in oil revenue owing to effective hedging contracts	$ 5.3	$ 36.3	$ 19.7	$ 96.8
Decrease/increase in gas revenue owing to effective hedging contracts	2.4	3.6	9.7	10.3
Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives	2.3	31.6	(1.7)	35.7
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion) (a)	7.7	39.9	29.4	107.1
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)	(0.2)	1.2	(0.7)	1.7
Designated as Hedging Instrument | Cash Flow Hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives	2.3	31.6	(1.7)	35.7
Designated as Hedging Instrument | Cash Flow Hedges | Derivative Income (Expense), Net
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)	(0.2)	1.2	(0.7)	1.7
Designated as Hedging Instrument | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income (Effective Portion) (a)	$ 7.7	$ 39.9	$ 29.4	$ 107.1