Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Beginning of Year	$ 5.1	$ 3.8	$ 7.2
Additions for Tax Positions of Current Year	3.1	1.5	0.2
Additions for Tax Positions of Prior Years	0.0	0.0	0.0
Reduction for Expiration of Federal Statute of Limitations	(0.1)	(0.2)	(3.6)
Balance at End of Year	8.1	5.1	3.8
Unrecognized tax benefit	7.6	4.6	3.3
Accrued interest included in unrecognized tax benefits	$ 0.5	$ 0.5	0.5
Interest on income taxes expense			$ 3.3