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Impairments Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Impaired Assets to be Disposed of by Method Other than Sale [Line Items]        
Asset Impairment Charges $ 44.0 $ 6.0 $ 21.0 $ 6,475.0
Exploration Expense $ 103.0 $ 73.0 [1],[2] 267.0 720.0 [1],[2]
Alaska Segment [Member] | Operating Segments [Member] | Point Thomson [Member]        
Impaired Assets to be Disposed of by Method Other than Sale [Line Items]        
Asset Impairment Charges       174.0
Lower Forty Eight Segment [Member] | Operating Segments [Member]        
Impaired Assets to be Disposed of by Method Other than Sale [Line Items]        
Asset Impairment Charges     55.0  
Lower Forty Eight Segment [Member] | Operating Segments [Member] | Barnett [Member]        
Impaired Assets to be Disposed of by Method Other than Sale [Line Items]        
Asset Impairment Charges       600.0
Lower Forty Eight Segment [Member] | Operating Segments [Member] | San Juan Properties [Member]        
Impaired Assets to be Disposed of by Method Other than Sale [Line Items]        
Asset Impairment Charges       3,300.0
Lower Forty Eight Segment [Member] | Operating Segments [Member] | GOM | Shenandoah [Member]        
Impaired Assets to be Disposed of by Method Other than Sale [Line Items]        
Exploration Expense       $ 51.0
Europe and North Africa Segment [Member] | Operating Segments [Member] | UK Field [Member]        
Impaired Assets to be Disposed of by Method Other than Sale [Line Items]        
Asset Impairment Charges     $ 49.0  
[1]
*Certain amounts have been reclassified to conform to the current-period presentation resulting from the adoption of ASU No. 2017-07.
[2]
See Note 2—Changes in Accounting Principles, for additional information.