Income Taxes (Deferred Tax Assets (Liabilities) Classification) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Other assets	$ 203.8	$ 162.1
Total deferred tax assets	203.8	162.1
Deferred tax liabilities:
Long-term income taxes	(22.3)	(22.2)
Total deferred tax liabilities	(22.3)	(22.2)
Net deferred tax assets	$ 181.5	$ 139.9