Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues [Abstract]
Earned Premiums	$ 1,063.6	$ 1,052.9	$ 658.1	$ 609.8	$ 605.9	$ 598.2	$ 582.5	$ 563.4	$ 3,384.4	$ 2,350.0	$ 2,220.0
Net Investment Income	91.3	92.0	78.4	79.2	82.6	85.9	77.1	81.6	340.9	327.2	298.3
Other Income	2.0	37.8	1.2	1.2	1.1	1.0	1.0	0.9	42.2	4.0	3.2
Equity Securities, FV-NI, Gain (Loss)	(76.4)	11.0	0.4	0.7					(64.3)	0.0	0.0
Debt and Equity Securities, Realized Gain (Loss)	16.4	3.6	3.8	2.6	11.5	8.1	26.4	10.5	26.4	56.5	33.1
Other-than-temporary Impairment Losses:
Total Other-than-temporary Impairment Losses	(2.2)	(1.8)	0.0	(0.5)	(3.7)	(2.9)	(2.6)	(5.2)	(4.5)	(14.4)	(33.0)
Portion of Losses Recognized in Other Comprehensive Income	0.0	0.0	0.0	0.0	(0.1)	0.0	0.0	0.2	0.0	0.1	0.3
Net Impairment Losses Recognized in Earnings	(2.2)	(1.8)	0.0	(0.5)	(3.8)	(2.9)	(2.6)	(5.0)	(4.5)	(14.3)	(32.7)
Total Revenues	1,094.7	1,195.5	741.9	693.0	697.3	690.3	684.4	651.4	3,725.1	2,723.4	2,521.9
Expenses [Abstract]
Policyholders’ Benefits and Incurred Losses and Loss Adjustment Expenses	772.8	757.3	499.5	436.9	472.5	440.1	447.4	477.4	2,466.5	1,837.4	1,780.8
Insurance Expenses	273.2	296.0	171.2	160.1	159.1	163.7	163.5	158.0	900.5	644.3	647.3
Interest and Other Expenses	42.6	61.7	25.7	29.0	21.5	18.2	21.4	19.5	159.0	80.6	90.3
Total Expenses	1,088.6	1,115.0	696.4	626.0	653.1	622.0	632.3	654.9	3,526.0	2,562.3	2,518.4
Income (Loss) from Continuing Operations before Income Taxes	6.1	80.5	45.5	67.0	44.2	68.3	52.1	(3.5)	199.1	161.1	3.5
Income Tax Benefit (Expense)	(1.1)	11.8	(8.0)	(13.4)	(8.3)	(20.5)	(15.5)	3.1	(10.7)	(41.2)	9.2
Income (Loss) from Continuing Operations	5.0	92.3	37.5	53.6	35.9	47.8	36.6	(0.4)	188.4	119.9	12.7
Income from Discontinued Operations	1.5	(0.1)	0.1	0.2	1.0	(0.1)	0.0	0.1	1.7	1.0	4.1
Net Income	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	$ 190.1	$ 120.9	$ 16.8
Income (Loss) from Continuing Operations Per Unrestricted Share [Abstract]
Income from Continuing Operations Per Unrestricted Share - Basic (usd pre share)	$ 0.08	$ 1.42	$ 0.73	$ 1.03	$ 0.69	$ 0.92	$ 0.71	$ (0.01)	$ 3.22	$ 2.32	$ 0.25
Income from Continuing Operations Per Unrestricted Share - Diluted (usd per share)	0.08	1.40	0.73	1.02	0.69	0.92	0.71	(0.01)	3.19	2.31	0.25
Net Income (Loss) Per Unrestricted Share [Abstract]
Net Income Per Unrestricted Share - Basic ( usd per share)	0.10	1.42	0.73	1.03	0.71	0.92	0.71	(0.01)	3.25	2.34	0.33
Net Income Per Unrestricted Share - Diluted (usd pre share)	0.10	1.40	0.73	1.02	0.71	0.92	0.71	(0.01)	3.22	2.33	$ 0.33
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