Business Segments - Segment Net Operating Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Segment Net Operating Income	$ 91.5	$ 36.5	$ 190.4	$ 94.0
Income from Continuing Operations	122.1	37.5	277.4	91.1
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income	81.2	43.1	186.7	104.1
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income	5.2	(6.0)	8.0	7.5
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income	62.7	22.4	142.3	45.8
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income	13.3	26.7	36.4	50.8
Corporate and Other
Segment Reporting Information [Line Items]
Segment Net Operating Income	10.3	(6.6)	3.7	(10.1)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	20.1	0.3	71.0	0.9
Net Realized Gains on Sales of Investments	16.8	3.0	29.5	5.1
Net Impairment Losses Recognized in Earnings	(5.3)	0.0	(8.1)	(0.4)
Acquisition Related Transaction, Integration and Other Costs	$ (1.0)	$ (2.3)	$ (5.4)	$ (8.5)