Acquisitions and Divestitures Schedule of Pro Forma Results Of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Revenues	$ 468.3	$ 507.6	$ 574.6	$ 468.1	$ 757.1	$ 858.5	$ 850.0	$ 827.6	$ 2,018.6	$ 3,293.2	$ 2,685.1
Net income (loss)	$ (38.6)	$ 21.1	$ (76.3)	$ (55.6)	$ 665.9	$ 171.1	$ (92.3)	$ 39.7	$ (149.4)	$ 784.4	$ 159.4
Earnings Per Share, Basic									$ (0.85)	$ 4.36	$ 0.89
Diluted total									$ (0.85)	$ 4.36	$ 0.89
Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Revenues										$ 3,293.2	$ 2,685.1
Net income (loss)										$ 784.4	$ 159.4
Earnings Per Share, Basic										$ 4.36	$ 0.89
Diluted total										$ 4.36	$ 0.89
Business Acquisition, Pro Forma Revenue										$ 3,319.3	$ 2,858.8
Business Acquisition, Pro Forma Net Income (Loss)										$ 791.4	$ 195.3
Business Acquisition, Pro Forma Earnings Per Share, Basic										$ 4.40	$ 1.09
Business Acquisition, Pro Forma Earnings Per Share, Diluted										$ 4.40	$ 1.09