Investments (Schedule of Pre-tax Realized and Unrealized Gains (Losses) on Equity Securities Recognized in Net Income) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Investments Debt And Equity Securities [Abstract]
Net losses recognized during the period	$ (23.0)
Less: net gains recognized on equity securities sold during the period	0.5
Net unrealized losses recognized during the period on equity securities still held	$ (23.5)