Reinsurance (Schedule of Trust Assets and Reinsurance Liabilities) (Details) - USD ($) $ in Millions		Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Reinsurance [Line Items]
Restricted cash and cash equivalents		$ 559.2	$ 457.7	$ 480.0	$ 410.0
Debt Securities, Available-for-sale, Restricted		99.9	93.2
Trust Assets [Member]
Reinsurance [Line Items]
Restricted cash and cash equivalents		3.0	1.4
Debt Securities, Available-for-sale, Restricted		99.9	93.2
Unearned Premium [Member]
Reinsurance [Line Items]
Accounts payable and accrued liabilities		76.9	67.6
Claims Reserve [Member]
Reinsurance [Line Items]
Accounts payable and accrued liabilities	[1]	$ 5.6	$ 5.6

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