Income Taxes - Schedule of Temporary Differences That Give Rise to Net Deferred Income Tax Assets (Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred Income Tax Assets:
Insurance Reserves	$ 18.4	$ 16.2
Unearned Premium Reserves	66.7	64.5
Tax Capitalization of Policy Acquisition Costs	46.6	44.6
Payroll and Employee Benefit Accruals	35.6	35.0
Net Operating Loss Carryforwards	1.1	3.3
Other	13.4	12.5
Total Deferred Income Tax Assets	181.8	176.1
Deferred Income Tax Liabilities:
Investments	258.8	155.6
Deferred Policy Acquisition Costs	123.7	112.9
Life VIF and P&C Customer Relationships	5.0	5.3
Goodwill and Other Intangible Assets Acquired	35.5	39.3
Depreciable Assets	42.1	37.6
Other	2.4	3.6
Total Deferred Income Tax Liabilities	467.5	354.3
Net Deferred Income Tax Liabilities	$ 285.7	$ 178.2