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, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ (32.6)	$ (71.1)	$ (32.6)
Core Commercial [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(27.6)	(27.5)	(27.3)
Specialty [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(28.2)	(18.5)	(10.5)
Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	22.0	(26.1)	1.0	$ (26.1)
Other [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ 1.2	$ 1.0	$ 4.2