Acquisitions & Divestitures Other Divestitures (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Divestitures [Line Items]
Proceeds from disposition of assets			$ 48.8	$ 2.3
Accounts Receivable, Net	$ 36.7		36.7
Net gain (loss) from asset sales, inclusive of restructuring costs	$ (3.9)	$ 19.8	(0.4)	19.8
Impairment of Oil and Gas Properties			404.4
Uinta Basin Divestiture [Member]
Divestitures [Line Items]
Proceeds from disposition of assets			155.0
Non-core properties Divestitures [Member]
Divestitures [Line Items]
Net gain (loss) from asset sales, inclusive of restructuring costs			0.7	$ 19.8
Uinta Basin Divestiture [Member]
Divestitures [Line Items]
Impairment of Oil and Gas Properties			$ 402.8