Schedule 3 - Supplementary Insurance Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Premiums		$ 3,384.4	$ 2,350.0	$ 2,220.0
Other Income		42.2	4.0	3.2
Net Investment Income		340.9	327.2	298.3
Insurance Claims and Policyholders' Benefits		2,466.5	1,837.4	1,780.8
Amortization of Deferred Policy Acquisition Costs		377.1	318.3	299.3
Other Insurance Expenses		523.4	326.0	348.0
Deferred Policy Acquisition Costs		470.0	365.3
Insurance Reserves		5,433.6	4,537.8
Unearned Premiums		1,424.3	653.9
Property and Casualty Insurance [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Premiums		730.7	730.3	741.5
Premiums Written		748.8	726.1	737.3
Other Income		0.0	0.0	0.0
Net Investment Income		61.8	58.9	48.6
Insurance Claims and Policyholders' Benefits		538.4	649.5	545.3
Amortization of Deferred Policy Acquisition Costs		117.2	114.6	115.3
Other Insurance Expenses		108.3	103.2	113.7
Deferred Policy Acquisition Costs		58.4	56.0
Insurance Reserves		452.9	461.9
Unearned Premiums		374.3	356.3
Specialty Property & Casualty Insurance [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Premiums		2,027.4	1,005.7	873.3
Premiums Written		2,067.4	1,043.5	883.6
Other Income		2.4	1.1	0.5
Net Investment Income		63.4	39.2	29.1
Insurance Claims and Policyholders' Benefits		1,523.8	800.5	773.9
Amortization of Deferred Policy Acquisition Costs		202.0	150.2	136.8
Other Insurance Expenses		219.7	14.8	19.9
Deferred Policy Acquisition Costs		121.4	40.2
Insurance Reserves		1,387.0	514.4
Unearned Premiums		1,025.0	269.3
Life and Health Insurance [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Premiums	[1]	626.3	614.0	605.2
Other Income	[1]	4.0	2.6	2.8
Net Investment Income	[1]	210.9	223.2	214.2
Insurance Claims and Policyholders' Benefits	[1]	404.2	387.4	461.6
Amortization of Deferred Policy Acquisition Costs	[1]	57.9	53.5	47.2
Other Insurance Expenses	[1]	263.2	258.7	266.7
Deferred Policy Acquisition Costs	[1]	290.2	269.1
Insurance Reserves	[1]	3,563.1	3,525.1
Unearned Premiums	[1]	25.0	28.3
Other Segments [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Premiums		0.0	0.0	0.0
Other Income		35.8	0.3	(0.1)
Net Investment Income		4.8	5.9	6.4
Insurance Claims and Policyholders' Benefits		0.1	0.0	0.0
Amortization of Deferred Policy Acquisition Costs		0.0	0.0	0.0
Other Insurance Expenses		(67.8)	(50.7)	$ (52.3)
Deferred Policy Acquisition Costs		0.0	0.0
Insurance Reserves		30.6	36.4
Unearned Premiums		$ 0.0	$ 0.0

[1] The Company’s Life & Health Insurance employee-agents also market certain property and casualty insurance products under common management. Accordingly, the Company includes the results of these property and casualty insurance products in its Life & Health Insurance segment.