Income Taxes - Textual (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Provision for income tax expense	$ 30.2	$ 43.1	$ 89.5	$ 111.4
Net deferred tax assets	82.6		82.6
Valuation allowance	37.1		37.1
Additional tax benefits due to approval of accounting method change					$ 51.3
Gross unrecognized tax benefits	0.9		0.9		0.3
Gross unrecognized tax benefits that would impact effective tax rate if recognized	0.9		0.9		$ 0.3
Amount of decrease in unrecognized tax benefit to be released	0.5		0.5
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Operating Loss Carryforwards	$ 72.0		$ 72.0