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Capitalized Exploratory Well Costs (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Capitalized Exploratory Well Costs [Line Items]        
Capitalized exploratory well costs charged to expense     $ 21.2 $ 0.0
Impairment $ 28.3 $ 5.0 28.4 $ 1,188.2
Central Basin Platform [Member]        
Capitalized Exploratory Well Costs [Line Items]        
Reclassifications to proved properties     (3.6)  
Capitalized exploratory well costs charged to expense     (21.2)  
Impairment     $ 28.3