Business Segments - Amortization of Deferred Policy Acquisition Costs By Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs	$ 408.3	$ 377.1	$ 318.3
Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs	224.9	202.0	150.2
Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs	120.1	117.2	114.6
Life & Health Insurance
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs	$ 63.3	$ 57.9	$ 53.5