Business Segments - Segment Information (Details) $ in Millions		3 Months Ended		9 Months Ended
	Sep. 26, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Operating Segments | segment				2
Earned Premiums		$ 1,052.9	$ 598.2	$ 2,320.8	$ 1,744.1
Net Investment Income		92.0	85.9	249.6	244.6
Other Income		37.8	1.0	40.2	2.9
Total Revenues		1,195.5	690.3	2,630.4	2,026.1
Income from Change in Fair Value of Equity Securities		11.0	0.0	12.1	0.0
Net Realized Gains on Sales of Investments		3.6	8.1	10.0	45.0
Net Impairment Losses Recognized in Earnings		(1.8)	(2.9)	(2.3)	(10.5)
Total Before Income Taxes		80.5	68.3	193.0	116.9
Income from Continuing Operations		92.3	47.8	183.4	84.0
Gain (Loss) Related to Litigation Settlement	$ 7.2
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)		28.7	(3.7)	18.0	(15.2)
Gain (Loss) Related to Litigation Settlement		28.2	0.0	28.2	0.0
Other Nonoperating Income (Expense)		(4.6)	(2.0)	(12.0)	(9.7)
Operating Income (Loss), Net of Tax		49.6	(2.0)	42.2	(9.7)
Tax Cuts and Jobs Act, Change in Tax Rate, Income Tax Expense (Benefit)		26.0	0.0	26.0	0.0
Gain (Loss) Related to Litigation Settlement		35.7	0.0	35.7	0.0
Other Nonoperating Income (Expense)		(7.0)	(3.7)	(17.7)	(15.2)
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Other Income		43.2	2.1	48.0	8.4
Income from Change in Fair Value of Equity Securities		11.0	0.0	12.1	0.0
Net Realized Gains on Sales of Investments		3.6	8.1	10.0	45.0
Net Impairment Losses Recognized in Earnings		(1.8)	(2.9)	(2.3)	(10.5)
Business Combination, Acquisition Related Costs		(28.2)	0.0	(38.0)	0.0
Change in Fair Value of Equity Securities, Net of Tax		8.7	0.0	9.6	0.0
Net Realized Gains (Losses) on Sales of Investments, Net of Tax		2.8	5.3	7.9	29.3
Net Impairment Losses Recognized in Earnings, Net of Tax		(1.4)	(1.9)	(1.8)	(6.8)
Acquisition Related Transaction, Integration and Other Costs		22.3	0.0	30.8	0.0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)		67.2	66.8	193.2	97.6
Total Revenues		1,139.5	683.0	2,562.6	1,983.2
Income from Continuing Operations		54.9	46.4	156.3	71.2
Operating Segments [Member] | Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)		34.2	30.8	96.9	(2.2)
Earned Premiums		895.2	443.5	1,851.5	1,283.1
Net Investment Income		34.0	27.8	79.4	72.5
Other Income		0.9	0.4	1.6	0.9
Total Revenues		930.1	471.7	1,932.5	1,356.5
Segment Net Operating Income (Loss)				79.4	5.7
Income from Continuing Operations		28.2	22.9
Operating Segments [Member] | Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)		33.0	36.0	96.3	99.8
Earned Premiums		157.7	154.7	469.3	461.0
Net Investment Income		50.5	55.9	157.9	163.8
Other Income		1.2	0.7	2.9	1.9
Total Revenues		209.4	211.3	630.1	626.7
Segment Net Operating Income (Loss)				$ 76.9	$ 65.5
Income from Continuing Operations		$ 26.7	$ 23.5