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Restricted Securities Available for Sale (Schedule of Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]    
Within one year, cost $ 2.8 $ 2.5
Within one year, fair value 2.8 2.5
Over one year to five years, cost 45.9 40.5
Over one year to five years, fair value 45.3 40.2
Over five years to ten years, cost 1.0 1.0
Over five years to ten years, fair value 1.0 1.0
Over ten years, cost 2.3 2.4
Over ten years, fair value 2.2 2.4
Total restricted securities available for sale, cost 52.0 46.4
Total restricted securities available for sale [1] $ 51.3 $ 46.1
[1] Measured and recorded at fair value on a recurring basis.