Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues [Abstract]
Earned Premiums	$ 605.9	$ 598.2	$ 582.5	$ 563.4	$ 561.4	$ 558.9	$ 553.7	$ 546.0	$ 2,350.0	$ 2,220.0	$ 2,009.6
Net Investment Income	82.6	85.9	77.1	81.6	79.9	77.7	73.7	67.0	327.2	298.3	302.6
Other Income	1.1	1.0	1.0	0.9	1.0	0.8	0.6	0.8	4.0	3.2	3.7
Net Realized Gains on Sales of Investments	11.5	8.1	26.4	10.5	9.1	11.6	5.6	6.8	56.5	33.1	52.1
Other-than-temporary Impairment Losses:
Total Other-than-temporary Impairment Losses	(3.7)	(2.9)	(2.6)	(5.2)	(8.7)	(8.3)	(6.4)	(9.6)	(14.4)	(33.0)	(27.4)
Portion of Losses Recognized in Other Comprehensive Income	(0.1)	0.0	0.0	0.2	0.0	0.0	0.0	0.3	0.1	0.3
Net Impairment Losses Recognized in Earnings	(3.8)	(2.9)	(2.6)	(5.0)	(8.7)	(8.3)	(6.4)	(9.3)	(14.3)	(32.7)	(27.2)
Total Revenues	697.3	690.3	684.4	651.4	642.7	640.7	627.2	611.3	2,723.4	2,521.9	2,340.8
Expenses [Abstract]
Policyholders’ Benefits and Incurred Losses and Loss Adjustment Expenses	472.5	440.1	447.4	477.4	418.3	490.2	436.1	436.2	1,837.4	1,780.8	1,467.6
Insurance Expenses	159.1	163.7	163.5	158.0	158.5	161.7	167.8	159.3	644.3	647.3	645.1
Interest and Other Expenses	21.5	18.2	21.4	19.5	25.3	22.0	20.7	22.3	80.6	90.3	107.6
Total Expenses	653.1	622.0	632.3	654.9	602.1	673.9	624.6	617.8	2,562.3	2,518.4	2,240.5
Income (Loss) from Continuing Operations before Income Taxes	44.2	68.3	52.1	(3.5)	40.6	(33.2)	2.6	(6.5)	161.1	3.5	100.3
Income Tax Benefit (Expense)	(8.3)	(20.5)	(15.5)	3.1	(11.5)	14.9	1.5	4.3	(41.2)	9.2	(20.1)
Income (Loss) from Continuing Operations	35.9	47.8	36.6	(0.4)	29.1	(18.3)	4.1	(2.2)	119.9	12.7	80.2
Income from Discontinued Operations	1.0	(0.1)	0.0	0.1	2.1	2.0	(0.1)	0.1	1.0	4.1	5.5
Net Income	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	$ 31.2	$ (16.3)	$ 4.0	$ (2.1)	$ 120.9	$ 16.8	$ 85.7
Income (Loss) from Continuing Operations Per Unrestricted Share [Abstract]
Income from Continuing Operations Per Unrestricted Share - Basic (usd pre share)	$ 0.69	$ 0.92	$ 0.71	$ (0.01)	$ 0.56	$ (0.36)	$ 0.08	$ (0.04)	$ 2.32	$ 0.25	$ 1.55
Income from Continuing Operations Per Unrestricted Share - Diluted (usd per share)	0.69	0.92	0.71	(0.01)	0.56	(0.36)	0.08	(0.04)	2.31	0.25	1.55
Net Income (Loss) Per Unrestricted Share [Abstract]
Net Income Per Unrestricted Share - Basic ( usd per share)	0.71	0.92	0.71	(0.01)	0.60	(0.32)	0.08	(0.04)	2.34	0.33	1.65
Net Income Per Unrestricted Share - Diluted (usd pre share)	0.71	0.92	0.71	(0.01)	0.60	(0.32)	0.08	(0.04)	2.33	0.33	1.65
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