Financial Instruments (Investment Portfolio Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Proceeds from sales	$ 5,769.3	$ 3,240.5	$ 4,733.3
Realized gross gains on sales	176.0	30.7	255.1
Realized gross losses on sales	$ 5.8	$ 14.6	$ 10.3