Acquisitions and Divestitures 2017 Permian Basin Acquisition (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) a	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Aggregate purchase price	$ 94.5	$ 54.6
2017 Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Aggregate purchase price	$ 720.7
Net acres of producing and undeveloped oil and gas properties | a	15,100
Additional acquisitions in the Permian Basin [Member]
Business Acquisition [Line Items]
Aggregate purchase price	$ 50.0