CONSOLIDATED STATEMENTS OF EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock, Value [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Other Comprehensive Income (Loss) [Member]	Stockholders' Equity, Total [Member]
Balance at Dec. 31, 2014		$ 1.8	$ (25.4)	$ 535.3	$ 3,587.9	$ (24.3)	$ 4,075.3
Shares at Dec. 31, 2014		176.2
Treasury shares at Dec. 31, 2014			(0.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 149.4	$ 0.0	$ 0.0	0.0	(149.4)	0.0	(149.4)
Net income (loss), shares		0.0	0.0
Dividends		0.0	0.0
Dividends		$ 0.0	$ 0.0	0.0	(14.1)	0.0	(14.1)
Share-based compensation		$ 0.0	$ 10.8	19.5	(6.1)	0.0	24.2
Share-based compensation, shares		1.1	0.3
Change in pension and postretirement liability, net of tax, shares		0.0	0.0
Change in pension and postretirement liability, net of tax		$ 0.0	$ 0.0	0.0	0.0	11.9	11.9
Shares at Dec. 31, 2015		177.3
Treasury shares at Dec. 31, 2015			(0.5)
Balance at Dec. 31, 2015		$ 1.8	$ (14.6)	554.8	3,418.3	(12.4)	3,947.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 1,245.0	$ 0.0	$ 0.0	0.0	(1,245.0)	0.0	(1,245.0)
Net income (loss), shares		0.0	0.0
Equity issuance, net of offering costs, shares		61.0	0.0
Equity issuance, net of offering costs		$ 0.6	$ 0.0	780.8	0.0	0.0	(781.4)
Share-based compensation		$ 0.0	$ (8.3)	31.0	0.0	0.0	22.7
Share-based compensation, shares		2.4	(0.6)
Change in pension and postretirement liability, net of tax, shares		0.0	0.0
Change in pension and postretirement liability, net of tax		$ 0.0	$ 0.0	0.0	0.0	(4.3)	(4.3)
Shares at Dec. 31, 2016		240.7
Treasury shares at Dec. 31, 2016	(1.1)		(1.1)
Balance at Dec. 31, 2016	$ 3,502.7	$ 2.4	$ (22.9)	1,366.6	2,173.3	(16.7)	3,502.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ (269.3)	$ 0.0	$ 0.0	0.0	269.3	0.0	269.3
Net income (loss), shares		0.0	0.0
Share-based compensation		$ 0.0	$ (11.3)	31.6	0.0	0.0	20.3
Share-based compensation, shares		2.3	(0.9)
Change in pension and postretirement liability, net of tax, shares		0.0	0.0
Change in pension and postretirement liability, net of tax		$ 0.0	$ 0.0	0.0	0.0	5.6	5.6
Shares at Dec. 31, 2017		243.0
Treasury shares at Dec. 31, 2017	(2.0)		(2.0)
Balance at Dec. 31, 2017		$ 2.4	$ (34.2)	$ 1,398.2	$ 2,442.6	$ (11.1)	$ 3,797.9