Income Taxes Income Taxes - Components of The Provision For Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Current income tax provision	$ 0.4	$ 2.5	$ 3.1
Deferred tax expense (benefit)	(197.2)	2.1	22.6
Total income tax provision (benefit)	$ (196.8)	$ 4.6	$ 25.7