Property and Casualty Insurance Reserves - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	$ 159.8	$ (14.6)	$ 106.7
Reinsurance recoverables	86.5	99.6
Provision for Expected Credit Losses	41.6	48.0
Write-offs of Uncollectible Receivables from Policyholders	$ (40.8)	(48.5)
Actuary estimate period used for ultimate and LAE	10 years
Personal Automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	$ 108.7	(17.6)	85.3
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	24.2	3.6	12.4
Preferred Automobile [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	21.6	1.8	12.1
Homeowners [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	1.5	(7.6)	(6.5)
Other Personal Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	3.8	5.2	3.4
Property and Liability Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	159.8	(14.6)	106.7
Reinsurance recoverables	27.8	39.6	41.9	$ 50.1
Catastrophe
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	$ (9.1)	$ (4.1)	$ (5.4)