Acquisitions and Divestitures 2016 Permian Basin Acquisition Pro Forma Results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Oil and Gas Revenue	$ 399.7	$ 382.4	$ 333.7	$ 261.3	$ 468.3	$ 507.6	$ 574.6	$ 468.1	$ 1,377.1	$ 2,018.6	$ 3,293.2
Net income (loss)	$ (133.3)	$ (50.9)	$ (197.0)	$ (863.8)	$ (38.6)	$ 21.1	$ (76.3)	$ (55.6)	$ (1,245.0)	$ (149.4)	$ 784.4
Earnings Per Share, Basic	$ (0.56)	$ (0.21)	$ (0.90)	$ (4.55)	$ (0.22)	$ 0.12	$ (0.43)	$ (0.32)	$ (5.62)	$ (0.85)	$ 4.36
Earnings Per Share, Diluted	$ (0.56)	$ (0.21)	$ (0.90)	$ (4.55)	$ (0.22)	$ 0.12	$ (0.43)	$ (0.32)	$ (5.62)	$ (0.85)	$ 4.36
2016 Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Oil and Gas Revenue									$ 1,377.1	$ 2,018.6
Net income (loss)									$ (1,245.0)	$ (149.4)
Earnings Per Share, Basic									$ (5.62)	$ (0.85)
Earnings Per Share, Diluted									$ (5.62)	$ (0.85)
Business Acquisition, Pro Forma Revenue									$ 1,392.5	$ 2,041.5
Business Acquisition, Pro Forma Net Income (Loss)									$ (1,246.8)	$ (152.5)
Business Acquisition, Pro Forma Earnings Per Share, Basic									$ (5.62)	$ (0.86)
Business Acquisition, Pro Forma Earnings Per Share, Diluted									$ (5.62)	$ (0.86)