Annual report pursuant to Section 13 and 15(d)

Supplemental Oil and Gas Disclosures - Cost Incurred in Oil and Gas Property Acquisition Exploration and Development Activities (Parenthetical) (Details)

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Supplemental Oil and Gas Disclosures - Cost Incurred in Oil and Gas Property Acquisition Exploration and Development Activities (Parenthetical) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Capitalized Costs Oil And Gas Producing Activities Net [Abstract]      
Additions (reductions) of asset retirement obligations $ 21.3 $ 10.8 $ (0.4)
Seismic costs 0.5 0.2 3.2
Geological and geophysical costs $ 4.2 $ 4.1 $ 5.7