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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Revenues	$ 410.5	$ 560.8	$ 532.4	$ 428.9	$ 429.0	$ 390.1	$ 383.7	$ 420.1	$ 1,932.6	$ 1,622.9	$ 1,377.1
Net income (loss)	$ (629.3)	$ 7.3	$ (336.0)	$ (53.6)	$ 150.3	$ (3.3)	$ 45.4	$ 76.9	$ (1,011.6)	$ 269.3	$ (1,245.0)
Earnings Per Share, Basic	$ (2.66)	$ 0.03	$ (1.42)	$ (0.22)	$ 0.62	$ (0.01)	$ 0.19	$ 0.32	$ (4.25)	$ 1.12	$ (5.62)
Diluted	$ (2.66)	$ 0.03	$ (1.42)	$ (0.22)	$ 0.62	$ (0.01)	$ 0.19	$ 0.32	$ (4.25)	$ 1.12	$ (5.62)
2016 Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Revenues											$ 1,377.1
Net income (loss)											$ (1,245.0)
Earnings Per Share, Basic											$ (5.62)
Diluted											$ (5.62)
Business Acquisition, Pro Forma Revenue											$ 1,392.5
Business Acquisition, Pro Forma Net Income (Loss)											$ (1,246.8)
Business Acquisition, Pro Forma Earnings Per Share, Basic											$ (5.62)
Business Acquisition, Pro Forma Earnings Per Share, Diluted											$ (5.62)