Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of (Favorable) Unfavorable Loss and LAE Reserve Development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ (15.9)	$ (32.6)	$ (71.1)
Workers' Compensation Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	35.4
Core Commercial [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(2.0)	(27.6)	(27.5)
Specialty [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(57.1)	(28.2)	(18.5)
Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	40.9	22.0	(26.1)
Other [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ 2.3	$ 1.2	$ 1.0