Acquisitions and Divestitures Pinedale Divestiture (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Divestitures [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment	$ 243.6	$ 806.8	$ 29.0
Net gain (loss) from asset sales, inclusive of restructuring costs	25.0	213.5	5.0
Results of Operations, Income before Income Taxes	(1,329.0)	(42.9)	(1,953.2)
Pinedale Divestiture [Member]
Divestitures [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment		718.2
Net gain (loss) from asset sales, inclusive of restructuring costs	1.2	180.4
Long-term Purchase Commitment, Amount	45.0
Purchase Commitment, Remaining Minimum Amount Committed	$ 9.3
Results of Operations, Income before Income Taxes		$ 251.0	(1,152.7)
Pinedale [Member]
Divestitures [Line Items]
Impairment of Oil and Gas Properties			1,164.0
Proved Property [Member] | Pinedale [Member]
Divestitures [Line Items]
Impairment of Oil and Gas Properties			$ 1,164.0