Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Jun. 30, 2024	Dec. 31, 2024
Schedule of Equity Method Investments [Line Items]
Surplus Notes and Guarantee Fund Certificates, Reciprocal Exchange	$ 29.0		$ 22.0
Surplus Notes and Guarantee Fund Certificates, Additional Reciprocal Exchange	7.0
Income on Investments in Alternative Energy Partnership	(0.6)	$ (6.0)
Income Tax Expense	22.8	16.4
Variable Interest Entity, Not Primary Beneficiary
Schedule of Equity Method Investments [Line Items]
Cash distribution from Investment	0.5	0.5
Income on Investments in Alternative Energy Partnership	0.5	0.4
Income Tax Expense	0.1	$ 0.1
Cash	2.7			$ 2.7
Equipment, Net of Depreciation	248.0			253.2
Other Assets	9.8			9.2
Assets	260.4			265.0
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	$ 17.3			$ 17.6