Income Taxes - Components of Income Before Tax and Benefit for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Valuation allowance	$ 158.8	$ 119.0
Components of income (loss) before income taxes
United States	(336.6)	(563.7)	$ (1,662.5)
Foreign	108.8	85.0	(31.7)
Net loss before income taxes	(227.8)	(478.7)	(1,694.2)
Current
U.S. Federal	5.0	10.2	(24.6)
U.S. State	(4.0)	(0.3)	(0.8)
Foreign	24.8	32.0	36.4
Total	25.8	41.9	11.0
Deferred
U.S. Federal	(5.8)	(129.5)	(287.4)
U.S. State	2.5	(8.5)	(22.0)
Foreign	(8.0)	(28.9)	(1.5)
Total	(11.3)	(166.9)	(310.9)
Total income tax expense (benefit)	$ 14.5	$ (125.0)	$ (299.9)