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Acquisitions & Divestitures Uinta Basin (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Sep. 30, 2018
Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Proceeds from Sale of Oil and Gas Property and Equipment $ 617.4 $ 33.3    
Noncurrent assets held for sale 0.0     $ 692.7
Gain (Loss) on Disposition of Oil and Gas Property (13.2) 3.5    
Uinta Basin Divestiture [Member]        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Proceeds from Sale of Oil and Gas Property and Equipment     $ 153.0  
Gain (Loss) on Disposition of Oil and Gas Property $ 2.2      
Uinta Basin [Member]        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Results of Operations, Loss before Income Taxes   $ (5.0)