Income Tax Expense - Gross Unrecognized Income Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gross Unrecognized Income Tax Benefits [Roll Forward]
Balance at January 1	$ 2.0	$ 2.4	$ 2.0
Additions for Tax Positions Related to the Current Year	0.1	0.1	0.5
Additions for Tax Positions Related to Prior Years	0.1	0.2	0.7
Reduction for Tax Positions Related to Prior Years	(0.1)	(0.3)	(0.7)
Lapse of Statute	(0.4)	(0.4)	(0.1)
Balance as of December 31	1.7	2.0	2.4
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	0.8
Unrecognized Tax Benefits, Accrued Interest	0.0	0.0	0.0
Unrecognized Tax Benefits, Penalties	0.0	$ 0.0	$ 0.0
Material Changes to Unrecognized Tax Benefits Expected During Next 12 Months	$ 0.0