Business Segments - Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Operating Segments | segment			2
Earned Premiums	$ 658.1	$ 582.5	$ 1,267.9	$ 1,145.9
Net Investment Income	78.4	77.1	157.6	158.7
Other Income	1.2	1.0	2.4	1.9
Total Revenues	741.9	684.4	1,434.9	1,335.8
Income from Change in Fair Value of Equity Securities	0.4	0.0	1.1	0.0
Net Realized Gains on Sales of Investments	3.8	26.4	6.4	36.9
Net Impairment Losses Recognized in Earnings	0.0	(2.6)	(0.5)	(7.6)
Total Before Income Taxes	45.5	52.1	112.5	48.6
Income from Continuing Operations	37.5	36.6	91.1	36.2
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Segment Operating Profit (Loss)	(6.7)	(7.1)	(10.7)	(11.5)
Segment Net Operating Income (Loss)	(5.1)	(4.4)	(7.4)	(7.7)
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Other Income	1.3	1.7	4.8	6.3
Income from Change in Fair Value of Equity Securities	0.4	0.0	1.1	0.0
Net Realized Gains on Sales of Investments	3.8	26.4	6.4	36.9
Net Impairment Losses Recognized in Earnings	0.0	(2.6)	(0.5)	(7.6)
Business Combination, Acquisition Related Costs	(3.6)	0.0	(9.8)	0.0
Change in Fair Value of Equity Securities, Net of Tax	0.3	0.0	0.9	0.0
Net Realized Gains (Losses) on Sales of Investments, Net of Tax	3.0	17.2	5.1	24.0
Net Impairment Losses Recognized in Earnings, Net of Tax	0.0	(1.6)	(0.4)	(4.9)
Acquisition Related Transaction and Integration Costs	2.3	0.0	8.5	0.0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	736.4	658.9	1,423.1	1,300.2
Segment Operating Profit (Loss)	51.6	35.4	126.0	30.8
Income from Continuing Operations	41.6	25.4	101.4	24.8
Operating Segments [Member] | Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums	501.1	429.2	956.3	839.6
Net Investment Income	22.9	20.6	45.4	44.7
Other Income	0.4	0.3	0.7	0.5
Total Revenues	524.4	450.1	1,002.4	884.8
Segment Operating Profit (Loss)	18.2	4.2	62.7	(33.0)
Segment Net Operating Income (Loss)			51.2	(17.2)
Income from Continuing Operations	15.2	4.9
Operating Segments [Member] | Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums	157.0	153.3	311.6	306.3
Net Investment Income	54.1	54.9	107.4	107.9
Other Income	0.9	0.6	1.7	1.2
Total Revenues	212.0	208.8	420.7	415.4
Segment Operating Profit (Loss)	33.4	31.2	63.3	63.8
Segment Net Operating Income (Loss)			$ 50.2	$ 42.0
Income from Continuing Operations	$ 26.4	$ 20.5