Restricted Securities Available for Sale (Schedule of Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Within one year, cost		$ 6.8	$ 1.1
Within one year, fair value		6.8	1.1
Over one year to five years, cost		58.3	58.1
Over one year to five years, fair value		59.4	60.0
Over five years to ten years, cost		1.7	4.7
Over five years to ten years, fair value		1.7	4.8
Over ten years, cost		0.2	0.2
Over ten years, fair value		0.2	0.2
Total restricted securities available for sale, cost		67.0	64.1
Total restricted securities available for sale	[1]	$ 68.1	$ 66.1

[1] Measured at fair value on a recurring basis.