Acquisitions & Divestitures (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) a	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) a	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Business Combinations [Abstract]
Proceeds from Sale of Oil and Gas Property and Equipment			$ 2.3	$ 23.7
Accounts Receivable, Net	$ 36.7		36.7
Net gain (loss) from asset sales	19.8	$ (0.8)	19.8	(0.3)
Business Acquisition [Line Items]
Acquisition Costs, Period Cost			76.6	29.8
Bargain purchase gain from acquisition			(0.4)	0.0
Goodwill, Acquired During Period			5.3	3.7
Williston & Permian Basins [Member]
Business Acquisition [Line Items]
Goodwill, Acquired During Period				$ 3.7
2016 Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Revenue of Acquiree since Acquisition Date, Actual	15.1		19.8
Earnings or Loss of Acquiree since Acquisition Date, Actual	$ 2.1		1.6
Payments to Acquire Businesses, Gross			$ 591.0
Gas and Oil Area, Developed, Net | a	9,600		9,600
Bargain purchase gain from acquisition					$ (18.2)