Short-term Investments (Gross Gains (Losses) Realized On Sales, Maturities And Other Securities Transactions Related To Investment Securities) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Investments [Abstract]
Proceeds from sales of available-for-sale investment securities	$ 464.7	$ 168.3	$ 2,842.8	$ 596.4
Gross gains	0.5	0.2	5.0	1.1
Gross losses	(3.2)	(0.1)	(3.6)	(0.2)
Net realized gains on sales of available-for-sale investment securities	$ (2.7)	$ 0.1	$ 1.4	$ 0.9