Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues [Abstract]
Earned Premiums	$ 561.4	$ 558.9	$ 553.7	$ 546.0	$ 541.5	$ 536.7	$ 500.1	$ 431.3	$ 2,220.0	$ 2,009.6	$ 1,862.2
Net Investment Income	79.9	77.7	73.7	67.0	79.4	75.9	76.7	70.6	298.3	302.6	309.1
Other Income	1.0	0.8	0.6	0.8	1.4	0.8	0.6	0.9	3.2	3.7	1.4
Net Realized Gains on Sales of Investments	9.1	11.6	5.6	6.8	9.4	5.3	34.0	3.4	33.1	52.1	39.1
Other-than-temporary Impairment Losses:
Total Other-than-temporary Impairment Losses	(8.7)	(8.3)	(6.4)	(9.6)	(14.9)	(3.3)	(2.2)	(7.0)	(33.0)	(27.4)	(15.2)
Portion of Losses Recognized in Other Comprehensive Income	0.0	0.0	0.0	(0.3)	0.2	0.0	0.0	0.0	0.3	0.2
Net Impairment Losses Recognized in Earnings	(8.7)	(8.3)	(6.4)	(9.3)	(14.7)	(3.3)	(2.2)	(7.0)	(32.7)	(27.2)	(15.2)
Total Revenues	642.7	640.7	627.2	611.3	617.0	615.4	609.2	499.2	2,521.9	2,340.8	2,196.6
Expenses [Abstract]
Policyholders’ Benefits and Incurred Losses and Loss Adjustment Expenses	418.3	490.2	436.1	436.2	416.0	378.8	375.1	297.7	1,780.8	1,467.6	1,261.7
Insurance Expenses	158.5	161.7	167.8	159.3	177.0	161.1	162.1	144.9	647.3	645.1	628.4
Write-off of Long-lived Assets					0.0	0.0	11.1	0.0	0.0	11.1	54.6
Loss from Early Extinguishment of Debt					0.0	0.0	0.0	9.1	0.0	9.1	0.0
Interest and Other Expenses	25.3	22.0	20.7	22.3	25.6	25.7	26.6	29.7	90.3	107.6	91.7
Total Expenses	602.1	673.9	624.6	617.8	618.6	565.6	574.9	481.4	2,518.4	2,240.5	2,036.4
Income (Loss) from Continuing Operations before Income Taxes	40.6	(33.2)	2.6	(6.5)	(1.6)	49.8	34.3	17.8	3.5	100.3	160.2
Income Tax Benefit (Expense)	(11.5)	14.9	1.5	4.3	2.9	(11.8)	(6.9)	(4.3)	9.2	(20.1)	(47.6)
Income (Loss) from Continuing Operations	29.1	(18.3)	4.1	(2.2)	1.3	38.0	27.4	13.5	12.7	80.2	112.6
Income from Discontinued Operations	2.1	2.0	(0.1)	0.1	3.3	(0.1)	2.3	0.0	4.1	5.5	1.9
Net Income	$ 31.2	$ (16.3)	$ 4.0	$ (2.1)	$ 4.6	$ 37.9	$ 29.7	$ 13.5	$ 16.8	$ 85.7	$ 114.5
Income (Loss) from Continuing Operations Per Unrestricted Share [Abstract]
Income from Continuing Operations Per Unrestricted Share - Basic (usd pre share)	$ 0.56	$ (0.36)	$ 0.08	$ (0.04)	$ 0.03	$ 0.73	$ 0.53	$ 0.26	$ 0.25	$ 1.55	$ 2.08
Income from Continuing Operations Per Unrestricted Share - Diluted (usd per share)	0.56	(0.36)	0.08	(0.04)	0.03	0.73	0.53	0.26	0.25	1.55	2.08
Net Income (Loss) Per Unrestricted Share [Abstract]
Net Income Per Unrestricted Share - Basic ( usd per share)	0.60	(0.32)	0.08	(0.04)	0.09	0.73	0.57	0.26	0.33	1.65	2.12
Net Income Per Unrestricted Share - Diluted (usd pre share)	0.60	(0.32)	0.08	(0.04)	0.09	0.73	0.57	0.26	0.33	1.65	2.12
Dividends Paid to Shareholders Per Share (usd per share)	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 0.96	$ 0.96	$ 0.96