Recently Issued Accounting Pronouncements (Cumulative Effect of Changes from Adoption of Topic 606) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Deferred commissions, current and non-current	$ 25.1	$ 22.7	$ 22.7
Liabilities:
Deferred income tax liability		5.7
Equity:
Retained earnings	1,114.8	17.0	958.7
Balances without adoption of Topic 606
Assets
Deferred commissions, current and non-current	0.0		0.0
Liabilities:
Deferred income tax liability			0.0
Equity:
Retained earnings			$ 0.0
Adjustments due to Topic 606
Assets
Deferred commissions, current and non-current	$ 25.1
Adjustments due to Topic 606 | ASU 2014-09
Assets
Deferred commissions, current and non-current		22.7
Liabilities:
Deferred income tax liability		5.7
Equity:
Retained earnings		$ 17.0