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Derivative Instruments, Hedging Activities and Fair Value Measurements, Level 3 Recurring Valuation Techniques (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
$ / bbl
Dec. 31, 2016
USD ($)
$ / bbl
Asset commodity derivatives - Crude oil [Member] | Liability commodity derivatives - Crude oil [Member] | Level 3 [Member]    
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]    
Fair Value Measurements, Valuation Techniques Discounted cash flow  
Input description Forward commodity prices  
Asset commodity derivatives - Crude oil [Member] | Liability commodity derivatives - Crude oil [Member] | Level 3 [Member] | Minimum [Member]    
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]    
Fair value inputs, forward commodity price (in dollars per unit) | $ / bbl 60.21 51.73
Asset commodity derivatives - Crude oil [Member] | Liability commodity derivatives - Crude oil [Member] | Level 3 [Member] | Maximum [Member]    
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]    
Fair value inputs, forward commodity price (in dollars per unit) | $ / bbl 66.05 54.77
Fair Value, Measurements, Recurring [Member]    
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]    
Commodity asset derivatives $ 161.7 $ 541.4
Commodity liability derivatives 176.9 739.4
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]    
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]    
Commodity asset derivatives 2.9 1.7
Commodity liability derivatives 1.7 0.3
Fair Value, Measurements, Recurring [Member] | Liability commodity derivatives - Crude oil [Member] | Level 3 [Member]    
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]    
Commodity liability derivatives 1.7 0.3
Fair Value, Measurements, Recurring [Member] | Asset commodity derivatives - Crude oil [Member] | Level 3 [Member]    
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]    
Commodity asset derivatives $ 2.9 $ 1.7