Schedule 3 - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	$ 4,672.2	$ 4,472.4	$ 3,384.4
Other Income	94.6	35.5	42.2
Net Investment Income	348.2	364.3	340.9
Insurance Claims and Policy- holders’ Benefits	3,323.6	3,188.3	2,466.5
Amortization of Deferred Policy Acquisition Costs	641.8	408.3	377.1
Other Insurance Expenses	458.7	611.4	540.5
Deferred Policy Acquisition Costs	589.3	537.7
Insurance Reserves	5,510.0	5,471.8
Unearned Premiums	1,615.1	1,545.5
Specialty Property & Casualty Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	3,335.3	3,078.4	2,027.4
Premiums Written	3,435.5	3,211.3	2,067.4
Other Income	1.8	7.0	2.4
Net Investment Income	114.1	107.5	63.4
Insurance Claims and Policy- holders’ Benefits	2,378.4	2,278.9	1,523.8
Amortization of Deferred Policy Acquisition Costs	497.5	224.9	202.0
Other Insurance Expenses	154.4	330.7	219.7
Deferred Policy Acquisition Costs	177.4	161.6
Insurance Reserves	1,544.8	1,551.0
Unearned Premiums	1,262.9	1,158.0
Preferred Property & Casualty Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	688.2	750.3	730.7
Premiums Written	653.0	739.3	748.8
Other Income	0.1	0.0	0.0
Net Investment Income	37.7	44.1	61.8
Insurance Claims and Policy- holders’ Benefits	503.1	508.8	538.4
Amortization of Deferred Policy Acquisition Costs	111.2	120.1	117.2
Other Insurance Expenses	109.9	113.2	108.3
Deferred Policy Acquisition Costs	51.2	57.7
Insurance Reserves	411.6	388.5
Unearned Premiums	329.2	363.4
Life & Health Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	648.7	643.7	626.3
Other Income	0.6	8.5	4.0
Net Investment Income	198.8	206.4	210.9
Insurance Claims and Policy- holders’ Benefits	442.0	402.7	404.2
Amortization of Deferred Policy Acquisition Costs	33.1	63.3	57.9
Other Insurance Expenses	301.8	270.7	263.2
Deferred Policy Acquisition Costs	360.7	318.4
Insurance Reserves	3,532.1	3,505.3
Unearned Premiums	23.0	24.1
Other
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	0.0	0.0	0.0
Other Income	92.1	20.0	35.8
Net Investment Income	(2.4)	6.3	4.8
Insurance Claims and Policy- holders’ Benefits	0.1	(2.1)	0.1
Amortization of Deferred Policy Acquisition Costs	0.0	0.0	0.0
Other Insurance Expenses	(107.4)	(103.2)	$ (50.7)
Deferred Policy Acquisition Costs	0.0	0.0
Insurance Reserves	21.5	27.0
Unearned Premiums	$ 0.0	$ 0.0