Acquisitions & Divestitures (Narrative) (Details) - Permian Basin [Member] $ in Millions	3 Months Ended	7 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($) a
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross				$ 941.8
Net acres of producing and undeveloped oil and gas properties | a				26,500
Number of vertical producing wells				270
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	$ 41.0	$ 114.2	$ 110.6
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	$ (1.9)	$ 23.1	$ (1.0)