Condensed Consolidated Financial Statements, Accounting Standards Updates (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
New Accounting Pronouncements [Abstract]
Right to use asset	$ 61.3
Operating lease liability	61.4
ASU 2016-02 [Member]
New Accounting Pronouncements [Abstract]
Right to use asset	61.3
Operating lease liability	$ 61.4
ASU 2018-02 [Member] | Accumulated Deficit [Member]
New Accounting Pronouncements [Abstract]
Cumulative effect adjustment		$ 8.2		$ 0.0
ASU 2018-02 [Member] | Accumulated Other Comprehensive Loss [Member]
New Accounting Pronouncements [Abstract]
Cumulative effect adjustment		$ (8.2)	[1]	$ 0.0

[1] All amounts are net of tax. Amounts in parentheses indicate debits.