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Asset Retirement Obligations (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Asset retirement obligation, current $ 5.8 $ 1.9
Asset retirement obligations, noncurrent 225.8 204.9
ARO Liability [Roll Forward]    
ARO liability, Beginning Balance 206.8 195.1
Accretion 8.9 8.7
Liabilities incurred 17.0 3.8
Revisions of Estimates 6.5 17.2
Liabilities related to assets sold 0.0 (16.0)
Liabilities settled (7.6) (2.0)
ARO liability, Ending Balance 231.6 $ 206.8
Permian Basin Acquisition [Member]    
ARO Liability [Roll Forward]    
Liabilities incurred $ 11.6