Reinsurance Reinsurance (Schedule of Trust Assets and Reinsurance Liabilities) (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Restricted cash and cash equivalents		$ 273.6	$ 224.7
Restricted securities available for sale	[1]	46.1	45.3
Accounts payable and accrued liabilities		140.5	143.9
Trust Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash and cash equivalents		0.7	0.5
Restricted securities available for sale		46.1	45.3
Unearned Premium [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts payable and accrued liabilities		33.5	32.8
Claims Reserve [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts payable and accrued liabilities	[2]	$ 1.1	$ 1.0

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

[2] The claims reserve represents our liability for incurred-but-not-reported claims and is estimated based on historical claims experience.