Reinsurance (Schedule of Effects of Reinsurance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reinsurance Disclosures [Abstract]
Property and casualty premiums written, Direct	$ 5,131.4	$ 5,090.8	$ 4,816.0
Property and casualty premiums written, Assumed	23.1	27.7	27.6
Property and casualty premiums written, Ceded	(556.0)	(536.8)	(458.8)
Property and casualty premiums written, Net	4,598.5	4,581.7	4,384.8
Property and casualty premiums earned, Direct	5,063.8	4,953.3	4,673.6
Property and casualty premiums earned, Assumed	24.6	26.3	26.6
Property and casualty premiums earned, Ceded	(561.0)	(505.1)	(445.8)
Premiums Earned, Net, Total	$ 4,527.4	$ 4,474.5	$ 4,254.4
Percentage of assumed to net premiums earned	0.50%	0.60%	0.60%
Property and casualty losses and LAE, Direct	$ 3,200.5	$ 3,223.3	$ 2,986.2
Property and casualty losses and LAE, Assumed	15.9	21.4	25.2
Property and casualty losses and LAE, Ceded	(371.2)	(379.2)	(286.8)
Policyholder Benefits and Claims Incurred, Net, Total	$ 2,845.2	$ 2,865.5	$ 2,724.6