Acquisitions & Divestitures Pinedale Divestiture (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Divestitures [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment			$ 787.9	$ 28.9
Net gain (loss) from asset sales	$ 185.4	$ 5.3	205.2	5.0
Minimum volume commitment			45.0
INCOME (LOSS) BEFORE INCOME TAXES	(6.5)	(79.9)	188.7	(1,752.9)
Total [Member]
Divestitures [Line Items]
Minimum volume commitment remaining	35.0		35.0
Short-term [Member]
Divestitures [Line Items]
Minimum volume commitment remaining	28.0		28.0
Long-term [Member]
Divestitures [Line Items]
Minimum volume commitment remaining	7.0		7.0
Pinedale Divestiture [Member]
Divestitures [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment			718.2
Net gain (loss) from asset sales			178.8
INCOME (LOSS) BEFORE INCOME TAXES	208.2	$ 19.2	251.2	(1,177.0)
Restructuring costs, expected cost	0.8		0.8
One-time Termination Benefits [Member] | Pinedale Divestiture [Member]
Divestitures [Line Items]
Restructuring costs, expected cost	0.5		0.5
Restructuring costs, cost incurred to date	0.5		0.5
Employee Relocation [Member] | Pinedale Divestiture [Member]
Divestitures [Line Items]
Restructuring costs, expected cost	$ 0.3		$ 0.3
Proved Property [Member]
Divestitures [Line Items]
Impairment				1,167.9
Proved Property [Member] | Pinedale [Member]
Divestitures [Line Items]
Impairment				$ 1,164.0