Property and Casualty Insurance Reserves - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	$ 36.4	$ (71.1)	$ (7.4)
Reinsurance recoverables	50.1	122.6
Specialty Automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	28.2	(23.8)	5.5
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	(12.9)	(12.9)	(6.1)
Preferred Automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	26.7	(8.2)	5.8
Homeowners
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	(2.1)	(19.7)	3.2
Other Personal Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	(3.5)	(6.5)	(4.3)
Property and Liability Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	36.4	(71.1)	(7.4)
Reinsurance recoverables	50.1	65.6	101.9	$ 53.1
Catastrophe
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	$ 0.2	$ (17.1)	$ (8.4)