Schedule 3 - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums2	$ 4,529.4	$ 5,213.4	$ 5,179.2
Other Income (Loss)	7.2	9.2	4.8
Net Investment Income	419.7	422.6	427.3
Insurance Claims and Policy- holders’ Benefits2	3,820.0	4,432.6	4,519.6
Amortization of Deferred Policy Acquisition Costs2	607.1	705.7	695.1
Other Insurance Expenses2	445.3	495.3	533.7
Deferred Policy Acquisition Costs2	591.6	635.6
Insurance Reserves2	6,102.9	6,033.1
Unearned Premiums	1,300.8	1,704.4
Specialty Property & Casualty Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums2	3,632.5	4,046.4	3,948.5
Premiums Written	3,305.4	3,934.4	4,057.3
Other Income (Loss)	4.5	6.0	4.1
Net Investment Income	168.3	140.7	152.5
Insurance Claims and Policy- holders’ Benefits2	3,141.9	3,578.2	3,593.7
Amortization of Deferred Policy Acquisition Costs2	496.2	569.8	546.7
Other Insurance Expenses2	245.1	232.1	227.8
Deferred Policy Acquisition Costs2	142.7	192.6
Insurance Reserves2	2,308.7	2,321.1
Unearned Premiums	1,104.5	1,431.5
Non-Core Operations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums2	509.3	595.5	651.7
Premiums Written	435.5	527.1	642.0
Other Income (Loss)	0.0	0.0	0.0
Net Investment Income	48.7	49.7	68.6
Insurance Claims and Policy- holders’ Benefits2	434.6	493.4	537.4
Amortization of Deferred Policy Acquisition Costs2	71.0	93.5	103.8
Other Insurance Expenses2	75.6	90.0	102.6
Deferred Policy Acquisition Costs2	22.0	38.1
Insurance Reserves2	356.4	419.1
Unearned Premiums	188.7	264.0
Life Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums2	387.6	571.5	579.0
Other Income (Loss)	(0.2)	(0.6)	(1.3)
Net Investment Income	193.4	216.5	202.7
Insurance Claims and Policy- holders’ Benefits2	243.4	360.8	388.5
Amortization of Deferred Policy Acquisition Costs2	39.9	42.4	44.6
Other Insurance Expenses2	235.9	300.9	325.0
Deferred Policy Acquisition Costs2	426.9	404.9
Insurance Reserves2	3,425.3	3,278.5
Unearned Premiums	7.6	8.9
Other
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums2	0.0	0.0	0.0
Other Income (Loss)	2.9	3.8	2.0
Net Investment Income	9.3	15.7	3.5
Insurance Claims and Policy- holders’ Benefits2	0.1	0.2	0.0
Amortization of Deferred Policy Acquisition Costs2	0.0	0.0	0.0
Other Insurance Expenses2	(111.3)	(127.7)	$ (121.7)
Deferred Policy Acquisition Costs2	0.0	0.0
Insurance Reserves2	12.5	14.4
Unearned Premiums	$ 0.0	$ 0.0