Reinsurance (Schedule of Trust Assets and Reinsurance Liabilities) (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Restricted cash and cash equivalents		$ 368.5	$ 255.6	$ 316.7	$ 224.7
Restricted securities available for sale	[1]	51.3	46.1
Accounts payable and accrued liabilities		168.9	151.7
Trust Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash and cash equivalents		1.5	0.8
Restricted securities available for sale		51.3	46.1
Unearned Premium [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts payable and accrued liabilities		40.0	34.1
Claims Reserve [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts payable and accrued liabilities	[2]	$ 1.0	$ 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.