Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Schedule of Equity Method Investments [Line Items]
Income Tax Expense (Benefit)	$ 16.4	$ (23.8)
Surplus Notes and Guarantee Fund Certificates, Reciprocal Exchange	6.0		$ 4.0
Surplus Notes and Guarantee Fund Certificates, Additional Reciprocal Exchange	2.0
Variable Interest Entity, Not Primary Beneficiary
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Distribution	0.5	0.5
Income from Change in Value of Alternative Energy Partnership Investments	0.4	0.7
Income Tax Credits and Adjustments	0.0	(0.1)
Income Tax Expense (Benefit)	(0.1)	$ (0.2)
Cash and Cash Equivalents, at Carrying Value	2.7		2.7
Property, Plant and Equipment, Net	255.0		256.2
Other Assets	7.5		7.5
Assets	265.2		266.4
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	$ 17.2		$ 17.3