Acquisitions and Divestitures Uinta Basin Divestiture (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 678.9	$ 243.6	$ 806.8
Gain (Loss) on Disposition of Oil and Gas Property	3.9	25.0	$ 213.5
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
Restructuring Charges		5.4
Impairment of Oil and Gas Properties		402.8
Gain (Loss) on Disposition of Oil and Gas Property	$ (0.2)	$ (12.6)