Investments (Proceeds and Realized Gains Losses From Investments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments [Abstract]
Proceeds	$ 3,384.6	$ 2,921.2	$ 9,840.4	$ 8,463.8
Gross realized gains	144.4	113.4	255.1	343.2
Gross realized losses	$ (17.2)	$ (13.0)	$ (67.5)	$ (136.7)