Financial Instruments (Schedule of Investment Portfolio Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Proceeds from sales	$ 264.8	$ 431.6	$ 5,529.0
Realized gross gains on sales	4.5	4.9	3.6
Realized gross losses on sales	$ 8.2	$ 3.0	$ 49.2