Restricted Securities Available for Sale (Schedule of Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Within one year, cost	$ 9.7	$ 3.8
Within one year, fair value	9.7	3.8
Over one year to five years, cost	38.8	45.1
Over one year to five years, fair value	39.0	45.0
Over five years to ten years, cost	1.9	4.1
Over five years to ten years, fair value	1.9	4.1
Over ten years, cost	2.4	0.3
Over ten years, fair value	2.4	0.3
Total restricted securities available for sale, cost	52.8	53.3
Total restricted securities available for sale	53.0	53.2
Estimated Fair Value [Member]
Summary Of Significant Accounting Policies [Line Items]
Total restricted securities available for sale	$ 53.0	$ 53.2