Investments (Schedule of Pre-tax Realized and Unrealized Gains (Losses) on Equity Securities Recognized in Net Income) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Investments Debt And Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ 6.0	$ (17.0)
Less: net losses recognized on equity securities sold during the period	(1.3)	(0.8)
Net unrealized gains (losses) recognized during the period on equity securities still held	$ 7.3	$ (16.2)