Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,085.3	$ 1,116.6	$ 2,251.7	$ 2,191.4
Net Investment Income	67.8	96.0	153.4	178.7
Other Income	1.5	22.7	91.8	24.6
Total Revenues	1,230.9	1,275.4	2,459.9	2,511.7
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	71.6	25.5	(46.2)	89.9
Impairment Losses			(19.0)	(10.3)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,151.5	1,213.4	2,408.4	2,371.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	759.0	766.0	1,581.5	1,495.3
Net Investment Income	16.9	28.9	45.7	50.4
Other Income	0.1	1.0	1.0	1.8
Total Revenues	776.0	795.9	1,628.2	1,547.5
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	163.6	188.5	344.5	374.1
Net Investment Income	4.3	12.3	14.0	20.6
Other Income	0.1	0.0	0.1	0.0
Total Revenues	168.0	200.8	358.6	394.7
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	162.7	162.1	325.7	322.0
Net Investment Income	44.3	53.0	95.3	104.7
Other Income	0.5	1.6	0.6	2.7
Total Revenues	207.5	216.7	421.6	429.4
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	71.6	25.5	(46.2)	89.9
Net Realized Gains on Sales of Investments	11.7	21.3	28.2	37.4
Impairment Losses	(7.0)	(6.7)	(19.0)	(10.3)
Other	$ 3.1	$ 21.9	$ 88.5	$ 23.1