Acquisitions & Divestitures (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) a	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Business Combinations [Abstract]
Proceeds from Sale of Oil and Gas Property and Equipment	$ 0.2	$ 22.9
Net gain (loss) from asset sales	0.0	0.5
Business Acquisition [Line Items]
Acquisition Costs, Period Cost	68.2	21.0
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	4.7
Earnings or Loss of Acquiree since Acquisition Date, Actual	1.1
Payments to Acquire Businesses, Gross	$ 591.0
Gas and Oil Area, Developed, Net | a	9,600
Bargain purchase gain from acquisition	$ 0.4		$ (18.2)