Reinsurance (Schedule of Effects of Reinsurance) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Disclosures [Abstract]
Property and casualty premiums written, Direct		$ 4,816.0	$ 4,540.0	$ 4,265.7
Property and casualty premiums written, Assumed		27.6	23.6	25.1
Property and casualty premiums written, Ceded		(458.8)	(454.5)	(408.1)
Property and casualty premiums written, Net		4,384.8	4,109.1	3,882.7
Property and casualty premiums earned, Direct		4,673.6	4,399.9	4,162.6
Property and casualty premiums earned, Assumed		26.6	23.6	24.4
Property and casualty premiums earned, Ceded		(445.8)	(443.1)	(397.5)
Premiums Earned, Net, Total		$ 4,254.4	$ 3,980.4	$ 3,789.5
Percentage of assumed to net premiums earned		0.60%	0.60%	0.60%
Property and casualty losses and LAE, Direct		$ 2,986.2	$ 2,934.6	$ 2,785.1
Property and casualty losses and LAE, Assumed		25.2	20.5	30.6
Property and casualty losses and LAE, Ceded	[1]	(286.8)	(375.5)	(269.7)
Policyholder Benefits and Claims Incurred, Net, Total		$ 2,724.6	$ 2,579.6	$ 2,546.0

[1] The higher level of ceded losses and LAE in 2017 as compared to 2016 and 2018 is primarily due to higher non-catastrophe large loss activity occurring in certain domestic lines