Restricted Securities Available for Sale (Schedule of Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) - USD ($) $ in Millions		Jun. 30, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Within one year, cost		$ 6.2	$ 5.7
Within one year, fair value		6.2	5.7
Over one year to five years, cost		55.7	50.8
Over one year to five years, fair value		57.9	51.8
Over five years to ten years, cost		3.5	1.5
Over five years to ten years, fair value		3.6	1.5
Over ten years, cost		0.2	0.3
Over ten years, fair value		0.2	0.3
Total restricted securities available for sale, cost		65.6	58.3
Total restricted securities available for sale	[1]	$ 67.9	$ 59.3

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