Schedule 3 - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	$ 5,266.3	$ 5,253.7	$ 4,672.2
Other Income	9.2	4.8	94.6
Net Investment Income	422.6	427.3	348.2
Insurance Claims and Policy- holders’ Benefits	4,504.4	4,600.8	3,323.6
Amortization of Deferred Policy Acquisition Costs	705.3	684.3	641.8
Other Insurance Expenses	495.3	533.8	458.7
Deferred Policy Acquisition Costs	625.6	677.6
Insurance Reserves	6,310.9	6,313.6
Unearned Premiums	1,704.4	1,898.7
Specialty Property & Casualty Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	4,046.4	3,948.5	3,335.3
Premiums Written	3,934.4	4,057.3	3,435.5
Other Income	6.0	4.1	1.8
Net Investment Income	140.7	152.5	114.1
Insurance Claims and Policy- holders’ Benefits	3,578.2	3,593.7	2,378.4
Amortization of Deferred Policy Acquisition Costs	569.8	546.7	497.5
Other Insurance Expenses	232.1	227.8	154.4
Deferred Policy Acquisition Costs	192.6	219.0
Insurance Reserves	2,321.1	2,319.7
Unearned Premiums	1,431.5	1,545.9
Preferred Property & Casualty Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	595.5	651.7	688.2
Premiums Written	527.1	642.0	653.0
Other Income	0.0	0.0	0.1
Net Investment Income	49.7	68.6	37.7
Insurance Claims and Policy- holders’ Benefits	493.4	537.4	503.1
Amortization of Deferred Policy Acquisition Costs	93.5	103.8	111.2
Other Insurance Expenses	90.0	102.6	109.9
Deferred Policy Acquisition Costs	38.1	49.2
Insurance Reserves	419.1	433.2
Unearned Premiums	264.0	331.6
Life & Health Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	624.4	653.5	648.7
Other Income	(0.6)	(1.3)	0.6
Net Investment Income	216.5	202.7	198.8
Insurance Claims and Policy- holders’ Benefits	432.6	469.7	442.0
Amortization of Deferred Policy Acquisition Costs	42.0	33.8	33.1
Other Insurance Expenses	300.9	325.1	301.8
Deferred Policy Acquisition Costs	394.9	409.4
Insurance Reserves	3,556.3	3,544.5
Unearned Premiums	8.9	21.2
Other
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	0.0	0.0	0.0
Other Income	3.8	2.0	92.1
Net Investment Income	15.7	3.5	(2.4)
Insurance Claims and Policy- holders’ Benefits	0.2	0.0	0.1
Amortization of Deferred Policy Acquisition Costs	0.0	0.0	0.0
Other Insurance Expenses	(127.7)	(121.7)	$ (107.4)
Deferred Policy Acquisition Costs	0.0	0.0
Insurance Reserves	14.4	16.2
Unearned Premiums	$ 0.0	$ 0.0