Accounting Policies - Recently Adopted Accounting Standards (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements
Non-current deferred tax asset		$ 1.6	$ 1.6
Noncurrent deferred tax liability		38.5	55.2
Long-term Debt		355.0	499.5
Unamortized debt issuance costs		$ 1.6	$ 2.4
Accounting Standards Update 2016-02 | Forecast | Minimum
New Accounting Pronouncements
Right-of-use asset	$ 25.0
Operating lease liability	25.0
Accounting Standards Update 2016-02 | Forecast | Maximum
New Accounting Pronouncements
Right-of-use asset	40.0
Operating lease liability	$ 40.0