Property and Casualty Insurance Reserves - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	$ (71.1)	$ (7.4)	$ 18.9
Reinsurance recoverables	122.6	158.3
Specialty Automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	23.8	5.5	(2.9)
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	(12.9)	(6.1)	(0.6)
Preferred Automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	(8.2)	(5.8)	(19.3)
Homeowners
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	(19.7)	3.2	(1.0)
Other Personal Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	(6.5)	(4.3)	3.7
Property and Liability Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	(71.1)	(7.4)	18.9
Reinsurance recoverables	65.6	101.9	53.1	$ 50.2
Catastrophe
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development	$ (17.1)	$ (8.4)	(4.5)
Catastrophe | Homeowners
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and LAE related to prior year (favorable) adverse development			$ (3.7)