Schedule 3 - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	$ 4,472.4	$ 3,384.4	$ 2,350.0
Other Income	35.5	42.2	4.0
Net Investment Income	364.3	340.9	327.2
Insurance Claims and Policy- holders’ Benefits	3,188.3	2,466.5	1,837.4
Amortization of Deferred Policy Acquisition Costs	408.3	377.1	318.3
Other Insurance Expenses	611.4	523.4	326.0
Deferred Policy Acquisition Costs	537.7	470.0
Insurance Reserves	5,471.8	5,366.8
Unearned Premiums	1,545.5	1,424.3
Specialty Property & Casualty Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	3,078.4	2,027.4	1,005.7
Premiums Written	3,211.3	2,067.4	1,043.5
Other Income	7.0	2.4	1.1
Net Investment Income	107.5	63.4	39.2
Insurance Claims and Policy- holders’ Benefits	2,278.9	1,523.8	800.5
Amortization of Deferred Policy Acquisition Costs	224.9	202.0	150.2
Other Insurance Expenses	330.7	219.7	14.8
Deferred Policy Acquisition Costs	161.6	121.4
Insurance Reserves	1,551.0	1,387.0
Unearned Premiums	1,158.0	1,025.0
Preferred Property & Casualty Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	750.3	730.7	730.3
Premiums Written	739.3	748.8	726.1
Other Income	0.0	0.0	0.0
Net Investment Income	44.1	61.8	58.9
Insurance Claims and Policy- holders’ Benefits	508.8	538.4	649.5
Amortization of Deferred Policy Acquisition Costs	120.1	117.2	114.6
Other Insurance Expenses	113.2	108.3	103.2
Deferred Policy Acquisition Costs	57.7	58.4
Insurance Reserves	388.5	452.9
Unearned Premiums	363.4	374.3
Life & Health Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	643.7	626.3	614.0
Other Income	8.5	4.0	2.6
Net Investment Income	206.4	210.9	223.2
Insurance Claims and Policy- holders’ Benefits	402.7	404.2	387.4
Amortization of Deferred Policy Acquisition Costs	63.3	57.9	53.5
Other Insurance Expenses	270.7	263.2	258.7
Deferred Policy Acquisition Costs	318.4	290.2
Insurance Reserves	3,505.3	3,496.3
Unearned Premiums	24.1	25.0
Other
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Earned Premiums	0.0	0.0	0.0
Other Income	20.0	35.8	0.3
Net Investment Income	6.3	4.8	5.9
Insurance Claims and Policy- holders’ Benefits	(2.1)	0.1	0.0
Amortization of Deferred Policy Acquisition Costs	0.0	0.0	0.0
Other Insurance Expenses	(103.2)	(67.8)	$ (50.7)
Deferred Policy Acquisition Costs	0.0	0.0
Insurance Reserves	27.0	30.6
Unearned Premiums	$ 0.0	$ 0.0