Business Segments - Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Operating Segments | segment			2
Earned Premiums	$ 582.5	$ 553.7	$ 1,145.9	$ 1,099.7
Net Investment Income	77.1	73.7	158.7	140.7
Other Income	1.0	0.6	1.9	1.4
Total Revenues	684.4	627.2	1,335.8	1,238.5
Net Realized Gains on Sales of Investments	26.4	5.6	36.9	12.4
Net Impairment Losses Recognized in Earnings	(2.6)	(6.4)	(7.6)	(15.7)
Segment Operating Profit (Loss)	28.3	3.4	19.3	(0.6)
Total Before Income Taxes	52.1	2.6	48.6	(3.9)
Segment Net Operating Income (Loss)	21.0	4.6	17.1	4.0
Income from Continuing Operations	36.6	4.1	36.2	1.9
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Segment Operating Profit (Loss)	(7.1)	(4.7)	(11.5)	(16.9)
Segment Net Operating Income (Loss)	(4.4)	(2.9)	(7.7)	(10.7)
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Other Income	1.7	3.8	6.3	3.9
Net Realized Gains on Sales of Investments	26.4	5.6	36.9	12.4
Net Impairment Losses Recognized in Earnings	(2.6)	(6.4)	(7.6)	(15.7)
Net Realized Gains (Losses) on Sales of Investments, Net of Tax	17.2	3.7	24.0	8.1
Net Impairment Losses Recognized in Earnings, Net of Tax	(1.6)	(4.2)	(4.9)	(10.2)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	658.9	624.2	1,300.2	1,237.9
Segment Operating Profit (Loss)	35.4	8.1	30.8	16.3
Income from Continuing Operations	25.4	7.5	24.8	14.7
Operating Segments [Member] | Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums	429.2	402.6	839.6	798.8
Net Investment Income	20.6	19.7	44.7	31.6
Other Income	0.3	0.1	0.5	0.3
Total Revenues	450.1	422.4	884.8	830.7
Segment Operating Profit (Loss)	4.2	(17.2)	(33.0)	(40.0)
Income from Continuing Operations	4.9	(8.9)	(17.2)	(22.0)
Operating Segments [Member] | Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums	153.3	151.1	306.3	300.9
Net Investment Income	54.9	50.1	107.9	105.1
Other Income	0.6	0.6	1.2	1.2
Total Revenues	208.8	201.8	415.4	407.2
Segment Operating Profit (Loss)	31.2	25.3	63.8	56.3
Income from Continuing Operations	$ 20.5	$ 16.4	$ 42.0	$ 36.7