Loss Per Share - Computation of Weighted Average Shares Outstanding (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Earnings Per Share [Line Items]
Net loss attributable to common stockholders	$ (141.5)	$ (69.2)	$ (61.1)	$ (24.4)	$ (1.9)	$ 104.7	$ (8.8)	$ (134.8)	$ (296.2)	$ (40.8)	$ (308.6)
Numerator adjustments for diluted loss per share:
Gain on settlement agreement related to Series B Convertible Preferred Stock									0.0	(103.0)	0.0
Gain on amendment of Series B Convertible Preferred Stock									0.0	(32.9)	0.0
Remeasurement adjustment associated with the Preferred Series B redemption liability									0.0	5.0	0.0
Loss allocated to PDH non-controlling interest									0.0	(5.9)	0.0
Net loss attributable to common stockholders for diluted loss per share									$ (296.2)	$ (177.6)	$ (308.6)
Basic weighted average common stock outstanding (in shares)									286.1	243.3	203.2
Denominator adjustments for diluted loss per share:
Share adjustments (in shares)									0.0	29.0	0.0
Dilutive weighted average common stock outstanding (in shares)									286.1	272.3	203.2
Loss per share attributable to common stockholders:
Basic (in dollars per share)	$ (0.49)	$ (0.24)	$ (0.21)	$ (0.09)	$ (0.01)	$ 0.45	$ (0.04)	$ (0.59)	$ (1.04)	$ (0.17)	$ (1.52)
Diluted (in dollars per share)	$ (0.49)	$ (0.24)	$ (0.21)	$ (0.09)	$ (0.01)	$ (0.15)	$ (0.04)	$ (0.59)	(1.04)	(0.65)	(1.52)
Dividends per share paid to common stockholders (in dollars per share)									$ 0	$ 0	$ 0
Series B Preferred Stock
Denominator adjustments for diluted loss per share:
Conversion related to the amendment of Series B Preferred Stock - assumed at beginning of reporting period (in shares)									0.0	15.3	0.0
Settlement of preferred stock redemption liability - assumed at beginning of reporting period (in shares)									0.0	5.7	0.0
Conversion of PDH non-controlling interest (in shares)									0.0	8.0	0.0