Reinsurance (Schedule of Effects of Reinsurance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reinsurance Disclosures [Abstract]
Property and casualty premiums written, Direct	$ 6,411.9	$ 6,024.4	$ 5,310.7
Property and casualty premiums written, Assumed	21.0	56.4	33.6
Property and casualty premiums written, Ceded	(622.7)	(604.3)	(350.9)
Property and casualty premiums written, Net	5,810.2	5,476.5	4,993.4
Property and casualty premiums earned, Direct	6,255.1	5,817.6	5,131.3
Property and casualty premiums earned, Assumed	37.4	47.6	27.7
Property and casualty premiums earned, Ceded	(629.4)	(612.9)	(388.8)
Premiums Earned, Net, Total	$ 5,663.1	$ 5,252.3	$ 4,770.2
Percentage of assumed to net premiums earned	0.70%	0.90%	0.60%
Property and casualty losses and LAE, Direct	$ 4,489.4	$ 3,940.8	$ 3,407.1
Property and casualty losses and LAE, Assumed	42.5	44.2	17.7
Property and casualty losses and LAE, Ceded	(397.3)	(361.6)	(290.6)
Policyholder Benefits and Claims Incurred, Net, Total	$ 4,134.6	$ 3,623.4	$ 3,134.2