Acquisitions & Divestitures Supplemental Proforma Results of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Business Acquisition [Line Items]
Total Revenues	$ 536.7	$ 910.0	$ 1,636.9	$ 2,614.7
Net income	$ 21.1	$ 171.1	$ (110.8)	$ 118.5
Earnings Per Share, Basic	$ 0.12	$ 0.95	$ (0.63)	$ 0.66
Earnings Per Share, Diluted	$ 0.12	$ 0.94	$ (0.63)	$ 0.66
Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Total Revenues				$ 2,614.7
Net income				$ 118.5
Earnings Per Share, Basic				$ 0.66
Earnings Per Share, Diluted				$ 0.66
Business Acquisition, Pro Forma Revenue				$ 2,640.8
Business Acquisition, Pro Forma Net Income (Loss)				$ 125.5
Business Acquisition, Pro Forma Earnings Per Share, Basic				$ 0.70
Business Acquisition, Pro Forma Earnings Per Share, Diluted				$ 0.70