Income Taxes - Textual (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2016
Income Tax Contingency [Line Items]
Provision for income tax expense	$ 43.1	$ 42.1	$ 111.4	$ 124.2
Net deferred tax assets	91.6		91.6
Valuation allowance	28.4		28.4
Additional tax benefits due to approval of accounting method change			55.0		$ 39.0
Gross unrecognized tax benefits	0.3		0.3			$ 2.8
Gross unrecognized tax benefits that would impact effective tax rate if recognized	0.3		0.3			$ 2.8
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Operating Loss Carryforwards	$ 62.0		$ 62.0