Business Segments - Segment Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 123.4	$ 70.4
Number of Operating Segments | segment	3
Earned Premiums	$ 1,074.8	609.8
Net Investment Income	82.7	79.2
Other Income	1.9	1.2
Total Revenues	1,236.3	693.0
Income from Change in Fair Value of Equity and Convertible Securities	64.4	0.7
Net Realized Gains on Sales of Investments	16.1	2.6
Net Impairment Losses Recognized in Earnings	(3.6)	(0.5)
Other Noncash Income (Expense)	1.2	2.0
Total Before Income Taxes	194.7	67.0
Segment Net Operating Income (Loss)	98.9	57.5
Change in Fair Value of Equity and Convertible Securities	50.9	0.6
Income from Continuing Operations	155.3	53.6
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	(8.6)	(5.5)
Operating Income (Loss), Net of Tax	(6.6)	(3.5)
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net Realized Gains on Sales of Investments	16.1	2.6
Net Impairment Losses Recognized in Earnings	(3.6)	(0.5)
Business Combination, Acquisition Related Costs	(5.6)	(6.2)
Net Realized Gains (Losses) on Sales of Investments, Net of Tax	12.7	2.1
Net Impairment Losses Recognized in Earnings, Net of Tax	2.8	0.4
Acquisition Related Transaction, Integration and Other Costs	(4.4)	(6.2)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	132.0	75.9
Total Revenues	1,158.2	688.2
Income from Continuing Operations	105.5	61.0
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	3.3	16.6
Earned Premiums	185.6	176.8
Net Investment Income	8.3	13.7
Total Revenues	193.9	190.5
Segment Net Operating Income (Loss)	2.8	13.5
Operating Segments [Member] | Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums		278.4
Net Investment Income		9.9
Other Income		0.3
Total Revenues	751.6	288.6
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	99.4	29.0
Earned Premiums	729.3
Net Investment Income	21.5
Other Income	0.8
Operating Income (Loss), Net of Tax	79.6	23.4
Operating Segments [Member] | Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	29.3	30.3
Earned Premiums	159.9	154.6
Net Investment Income	51.7	53.7
Other Income	1.1	0.8
Total Revenues	212.7	209.1
Segment Net Operating Income (Loss)	23.1	24.1
Homeowners [Member] | Operating Segments [Member] | Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums	60.3
Other Personal Lines [Member] | Operating Segments [Member] | Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums	9.9	10.1
Operating Segments [Member] | Homeowners [Member] | Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums		61.8
Operating Segments [Member] | Preferred Automobile [Member] | Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums	$ 115.4	$ 104.9