Schedule 3 - Supplementary Insurance Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Premiums		$ 2,350.0	$ 2,220.0	$ 2,009.6
Other Income		4.0	3.2	3.7
Net Investment Income		327.2	298.3	302.6
Insurance Claims and Policyholders' Benefits		1,837.4	1,780.8	1,467.5
Amortization of Deferred Policy Acquisition Costs		318.3	299.3	257.4
Other Insurance Expenses		325.9	348.0	387.8
Deferred Policy Acquisition Costs		365.3	332.0
Insurance Reserves		4,537.8	4,406.7
Unearned Premiums		653.9	618.7
Property and Casualty Insurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Premiums		1,736.0	1,614.8	1,415.2
Premiums Written		1,769.6	1,620.9	1,406.2
Other Income		1.1	0.5	0.6
Net Investment Income		94.3	72.4	73.3
Insurance Claims and Policyholders' Benefits		1,450.0	1,319.2	1,086.2
Amortization of Deferred Policy Acquisition Costs		264.8	252.1	213.1
Other Insurance Expenses		118.0	133.6	155.0
Deferred Policy Acquisition Costs		96.2	82.1
Insurance Reserves		976.3	884.1
Unearned Premiums		625.6	592.0
Life and Health Insurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Premiums	[1]	614.0	605.2	594.4
Other Income	[1]	2.6	2.8	2.4
Net Investment Income	[1]	221.5	213.2	214.2
Insurance Claims and Policyholders' Benefits	[1]	387.4	461.6	381.3
Amortization of Deferred Policy Acquisition Costs	[1]	53.5	47.2	44.3
Other Insurance Expenses	[1]	258.7	266.7	275.7
Deferred Policy Acquisition Costs	[1]	269.1	249.9
Insurance Reserves	[1]	3,525.1	3,479.8
Unearned Premiums	[1]	28.3	26.7
Other Segments [Member]
Supplementary Insurance Information, by Segment [Line Items]
Premiums		0.0	0.0	0.0
Other Income		0.3	(0.1)	0.7
Net Investment Income		11.4	12.7	15.1
Insurance Claims and Policyholders' Benefits		0.0	0.0	0.0
Amortization of Deferred Policy Acquisition Costs		0.0	0.0	0.0
Other Insurance Expenses		(50.8)	(52.3)	$ (42.9)
Deferred Policy Acquisition Costs		0.0	0.0
Insurance Reserves		36.4	42.8
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.