Acquisitions & Divestitures (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) a	Jun. 30, 2015 USD ($)
Business Acquisition [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment			$ 23.7	$ (2.4)
Gain (Loss) on Disposition of Property Plant Equipment	$ (0.8)	$ 24.5
Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Acquisition Costs, Period Cost			$ 600.0
Net Acres | a			9,400.0
Vertical Wells			96
Williston & Permian Basins [Member]
Business Acquisition [Line Items]
Acquisition Costs, Period Cost			$ 29.8
Goodwill, Acquired During Period			3.7
Other Southern Divestiture [Member]
Business Acquisition [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment			23.7
Gain (Loss) on Disposition of Property Plant Equipment			$ (0.3)
2014 Divestitures [Member]
Business Acquisition [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment				$ (6.0)