Business Segments - Segment Net Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net Operating Income	$ 258.4	$ 92.5	$ 12.4
Change in Fair Value of Equity and Convertible Securities	(50.8)	0.0	0.0
Partial Satisfaction of Judgment	28.2	0.0	0.0
Income (Loss) from Continuing Operations	188.4	119.9	12.7
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Operating Income	233.0	102.8	32.1
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Net Operating Income	25.7	(45.4)	15.6
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss), Net of Tax	115.8	56.3	(14.5)
Operating Segments [Member] | Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Net Operating Income	91.5	91.9	31.0
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Net Operating Income	25.4	(10.3)	(19.7)
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit)	26.4	7.4	0.0
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net Realized Gains on Sales of Investments	20.9	36.7	21.5
Net Impairment Losses Recognized in Earnings	3.6	9.3	21.2
Loss from Early Extinguishment of Debt	36.5	0.0	0.0
Excluding amounts attributable to Tax Law Changes [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Net Operating Income	$ (29.2)	$ (17.7)	$ (19.7)