Acquisition of Business (Purchase Price Allocation) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Jul. 02, 2018
Business Acquisition [Line Items]
Goodwill	$ 1,111.5	$ 1,112.4
Infinity Property and Casualty Corporation [Member]
Business Acquisition [Line Items]
Investments			$ 1,569.3
Short Term Investments			98.8
Receivables from Policyholders			4.0
Other Receivables			583.4
Other Receivables			31.7
Value of Intangible Assets Acquired (Reported in Other Assets)			262.7
Goodwill			788.5
Other Assets			102.1
Property and Casualty Insurance Reserves			(713.6)
Unearned Premiums			(715.6)
Debt			(282.1)
Deferred Income Tax Liabilities			(11.7)
Accrued Expenses and Other Liabilities			(169.6)
Total Purchase Price			1,548.7
Current Income Tax Asset [Member] | Infinity Property and Casualty Corporation [Member]
Business Acquisition [Line Items]
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets			$ 0.8