Acquisitions and Divestitures Schedule Of Purchase Accounting Entries (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Aggregate purchase price	$ 941.8
Recognized identifiable assets acquired and liabilities assumed, net	941.8
Permian Basin Acquisition [Member] | Proved Property [Member]
Business Acquisition [Line Items]
Recognized identifiable assets acquired and liabilities assumed, net	472.1
Permian Basin Acquisition [Member] | Unproved Property [Member]
Business Acquisition [Line Items]
Recognized identifiable assets acquired and liabilities assumed, net	480.6
Permian Basin Acquisition [Member] | Asset Retirement Obligation Costs [Member]
Business Acquisition [Line Items]
Recognized identifiable assets acquired and liabilities assumed, net	(9.7)
Permian Basin Acquisition [Member] | Other Liabilities [Member]
Business Acquisition [Line Items]
Recognized identifiable assets acquired and liabilities assumed, net	(1.2)
Williston Basin Acquisition [Member]
Business Acquisition [Line Items]
Aggregate purchase price		$ 1,392.7