Business Segments - Amortization of Deferred Policy Acquisitions Costs By Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Amortization of Deferred Policy Acquisition Costs	$ 318.3	$ 299.3	$ 257.4
Operating Segments [Member]
Segment Reporting Information [Line Items]
Amortization of Deferred Policy Acquisition Costs	318.3	299.3	257.4
Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Amortization of Deferred Policy Acquisition Costs	264.8	252.1	213.1
Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Amortization of Deferred Policy Acquisition Costs	$ 53.5	$ 47.2	$ 44.3