Business Segments - Segment Net Operating Income (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Segment Net Operating Income	$ 162.9	$ 98.9
Income from Continuing Operations	64.0	155.3
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income	100.8	105.5
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income	60.1	79.6
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income	18.4	2.8
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income	22.3	23.1
Corporate and Other
Segment Reporting Information [Line Items]
Segment Net Operating Income	62.1	(6.6)
Partial Satisfaction of Judgment			$ 70.6	$ 0.0
Other			$ (8.5)	$ (6.6)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	(93.1)	50.9
Net Realized Gains on Sales of Investments	13.0	12.7
Impairment Losses	(9.5)	(2.8)
Acquisition Related Transaction, Integration and Other Costs	$ (9.3)	$ (4.4)