Restricted Securities Available for Sale (Schedule of Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Within one year, cost		$ 0.7	$ 1.6
Within one year, fair value		0.7	1.6
Over one year to five years, cost		42.5	39.3
Over one year to five years, fair value		42.5	39.1
Over five years to ten years, cost		0.6	2.2
Over five years to ten years, fair value		0.6	2.2
Over ten years, cost		2.3	2.4
Over ten years, fair value		2.3	2.4
Total restricted securities available for sale, cost		46.1	45.5
Total restricted securities available for sale	[1]	$ 46.1	$ 45.3

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