Income Taxes (Details of Income Taxes) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Income before income taxes	$ 41.4	$ 140.6
Income tax provision	$ 7.0	$ 31.8
Effective tax rate	16.90%	22.60%
Discrete benefit (in percentage)	4.40%
Unrecognized tax benefits	$ 263.1
Unrecognized tax benefits could decrease up to	$ 45.8