Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Schedule of Equity Method Investments [Line Items]
Surplus Notes and Guarantee Fund Certificates, Reciprocal Exchange			$ 21.0		$ 4.0
Surplus Notes and Guarantee Fund Certificates, Additional Reciprocal Exchange			17.0
Income Tax Expense (Benefit)	$ 18.5	$ (44.4)	52.4	$ (87.0)
Variable Interest Entity, Not Primary Beneficiary
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Distribution	0.5	0.5	1.5	1.5
Income (Loss) from Equity Method Investments	0.5	0.8	1.5	2.3
Income Tax Credits and Adjustments	0.0	0.0	0.0	(0.1)
Income Tax Expense (Benefit)	(0.1)	$ (0.2)	(0.3)	$ (0.5)
Cash	2.6		2.6		2.7
Equipment, Net of Depreciation	251.5		251.5		256.2
Other Assets	9.5		9.5		7.5
Assets	263.6		263.6		266.4
Maximum Loss Exposure	$ 17.4		$ 17.4		$ 17.3