Business Segments - Amortization of Deferred Policy Acquisitions Costs By Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Amortization of Deferred Policy Acquisition Costs	$ 377.1	$ 318.3	$ 299.3
Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Amortization of Deferred Policy Acquisition Costs	117.2	114.6	115.3
Operating Segments [Member]
Segment Reporting Information [Line Items]
Amortization of Deferred Policy Acquisition Costs	377.1	318.3	299.3
Specialty Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Amortization of Deferred Policy Acquisition Costs	202.0	150.2	136.8
Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Amortization of Deferred Policy Acquisition Costs	$ 57.9	$ 53.5	$ 47.2