Income Taxes - Schedule of Temporary Differences That Give Rise to Net Deferred Income Tax Assets (Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Deferred Income Tax Assets:
Unearned Premium Reserves	$ 54.0	$ 70.8
Tax Capitalization of Policy Acquisition Costs	46.3	45.9
Payroll and Employee Benefit Accruals	34.8	32.8
Investments	103.4	152.2
Net Operating Loss and Credit Carryforwards	114.6	49.7
Other	32.2	22.6
Subtotal	385.3	374.0
Valuation Allowance	(27.4)	0.0
Total Deferred Income Tax Assets	357.9	374.0
Deferred Income Tax Liabilities:
Insurance Reserves	12.7	24.9
Deferred Policy Acquisition Costs	124.3	133.3
Life VIF and P&C Customer Relationships	3.2	3.7
Goodwill and Other Intangible Assets Acquired	32.6	36.9
Depreciable Assets	19.3	35.7
Other	6.0	10.5
Total Deferred Income Tax Liabilities	198.1	245.0
Net Deferred Income Tax (Assets)	$ 159.8	$ 129.0