Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Earned Premiums	$ 1,145.8	$ 1,135.2	$ 1,116.6	$ 1,074.8	$ 1,063.6	$ 1,052.9	$ 658.1	$ 609.8	$ 4,472.4	$ 3,384.4	$ 2,350.0
Net Investment Income	93.9	91.7	96.0	82.7	91.3	92.0	78.4	79.2	364.3	340.9	327.2
Other Income	3.7	7.2	22.7	1.9	2.0	37.8	1.2	1.2	35.5	42.2	4.0
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	39.2	9.8	25.5	64.4	(76.4)	11.0	0.4	0.7	138.9	(64.3)	0.0
Net Realized Gains on Sales of Investments	2.8	1.7	21.3	16.1	16.4	3.6	3.8	2.6	41.9	26.4	56.5
Other-than-temporary Impairment Losses:
Total Other-than-temporary Impairment Losses	(1.7)	(1.8)	(6.7)	(3.5)	(2.2)	(1.8)	0.0	(0.5)	(13.7)	(4.5)	(14.4)
Portion of Gains (Losses) Recognized in Other Comprehensive Income	0.0	0.0	0.0	(0.1)	0.0	0.0	0.0	0.0	(0.1)	0.0	0.1
Net Impairment Losses Recognized in Earnings	(1.7)	(1.8)	(6.7)	(3.6)	(2.2)	(1.8)	0.0	(0.5)	(13.8)	(4.5)	(14.3)
Total Revenues	1,283.7	1,243.8	1,275.4	1,236.3	1,094.7	1,195.5	741.9	693.0	5,039.2	3,725.1	2,723.4
Expenses:
Policyholders’ Benefits and Incurred Losses and Loss Adjustment Expenses	814.9	782.6	825.4	765.4	772.8	757.3	499.5	436.9	3,188.3	2,466.5	1,837.4
Insurance Expenses	265.4	256.0	263.5	234.8	273.2	296.0	171.2	160.1	1,019.7	900.5	644.3
Loss from Early Extinguishment of Debt	0.0	5.8	0.0	0.0					5.8	0.0	0.0
Interest and Other Expenses	46.5	37.9	38.0	41.4	42.6	61.7	25.7	29.0	163.8	159.0	80.6
Total Expenses	1,126.8	1,082.3	1,126.9	1,041.6	1,088.6	1,115.0	696.4	626.0	4,377.6	3,526.0	2,562.3
Income from Continuing Operations before Income Taxes	156.9	161.5	148.5	194.7	6.1	80.5	45.5	67.0	661.6	199.1	161.1
Income Tax Expense	(32.2)	(32.5)	(26.4)	(39.4)	(1.1)	11.8	(8.0)	(13.4)	(130.5)	(10.7)	(41.2)
Income from Continuing Operations					5.0	92.3	37.5	53.6	531.1	188.4	119.9
Income from Discontinued Operations					1.5	(0.1)	0.1	0.2	0.0	1.7	1.0
Net Income	$ 124.7	$ 129.0	$ 122.1	$ 155.3	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 531.1	$ 190.1	$ 120.9
Income from Continuing Operations Per Unrestricted Share:
Basic (in dollars per share)					$ 0.08	$ 1.42	$ 0.73	$ 1.03	$ 8.04	$ 3.22	$ 2.32
Diluted (in dollars per share)					0.08	1.40	0.73	1.02	7.96	3.19	2.31
Net Income Per Unrestricted Share:
Basic (in dollars per share)	$ 1.87	$ 1.93	$ 1.87	$ 2.38	0.10	1.42	0.73	1.03	8.04	3.25	2.34
Diluted (in dollars per share)	$ 1.85	$ 1.91	$ 1.84	$ 2.35	$ 0.10	$ 1.40	$ 0.73	$ 1.02	$ 7.96	$ 3.22	$ 2.33