Financial Instruments (Schedule of Investment Portfolio Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Proceeds from sales	$ 655.5	$ 5,668.0	$ 5,769.3
Realized gross gains on sales	40.0	11.8	176.0
Realized gross losses on sales	$ 7.9	$ 51.3	$ 5.8