Income Taxes - Schedule of Temporary Differences That Give Rise to Net Deferred Income Tax Assets (Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred Income Tax Assets:
Insurance Reserves	$ 17.0	$ 55.7
Unearned Premium Reserves	59.1	26.7
Tax Capitalization of Policy Acquisition Costs	44.0	41.9
Payroll and Employee Benefit Accruals	49.6	37.2
Net Operating Loss Carryforward	5.5	31.8
Other	11.2	3.7
Total Deferred Income Tax Assets	186.4	197.0
Deferred Income Tax Liabilities:
Investments	33.2	90.6
Deferred Policy Acquisition Costs	98.7	76.8
Life VIF and P&C Customer Relationships	6.0	6.9
Goodwill and Other Intangible Assets Acquired	45.2	22.3
Depreciable Assets	27.1	12.1
Other	2.4	3.1
Deferred Tax Liabilities, Gross	212.6	211.8
Total Deferred Income Tax Liabilities	$ (26.2)	$ (14.8)