Restricted Securities Available for Sale (Schedule of Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Restricted Securities Available For Sale [Line Items]
Within one year, cost		$ 2.5	$ 2.5
Within one year, fair value		2.5	2.5
Over one year to five years, cost		53.0	40.5
Over one year to five years, fair value		52.2	40.2
Over five years to ten years, cost		0.6	1.0
Over five years to ten years, fair value		0.6	1.0
Over ten years, cost		1.4	2.4
Over ten years, fair value		1.4	2.4
Total restricted securities available for sale, cost		57.5	46.4
Total restricted securities available for sale	[1]	$ 56.7	$ 46.1

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