Investments (Proceeds and Realized Gains and Losses From Investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments [Abstract]
Proceeds	$ 11,779.8	$ 10,255.9	$ 13,662.8
Gross realized gains	509.1	306.1	522.7
Gross realized losses	$ (351.6)	$ (129.1)	$ (250.8)