SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Net incurred losses and LAE in respect of losses occurring in current year		$ 4,150.5	$ 3,656.0	$ 3,205.3
Net incurred losses and LAE in respect of losses occurring in prior years		15.9	32.6	71.1
Reinsurance Recoverable On Unpaid Losses Property Casualty Liability		1,795.0	1,748.6	1,693.8	$ 1,641.6
Prepaid premiums, gross		99.1	104.9
Consolidated Property and Casualty Insurance Entity [Member]
Deferred acquisition costs		620.8	604.8
Reserves for unpaid claims and claim adjustment expenses	[1]	7,308.1	7,012.6
Unearned premiums	[1]	3,102.5	2,954.2
Earned premiums		5,663.1	5,252.3	4,770.2
Net investment income		332.1	296.3	310.7
Net incurred losses and LAE in respect of losses occurring in current year		4,150.5	3,656.0	3,205.3
Net incurred losses and LAE in respect of losses occurring in prior years		(15.9)	(32.6)	(71.1)
Amortization of deferred acquisition costs		1,176.0	1,093.2	982.7
Paid claims and claim adjustment expenses		3,885.5	3,113.2	2,762.8
Premiums written		$ 5,810.2	$ 5,476.5	$ 4,993.4

[1] Reserves for unpaid claims and claim adjustment expenses are shown gross of $1,795.0 million and $1,748.6 million of reinsurance recoverable on unpaid losses in 2023 and 2022, respectively. Unearned premiums are shown gross of prepaid premiums of $99.1 million and $104.9 million in 2023 and 2022, respectively. Reserves for unpaid claims and claims adjustment expense also include policyholder dividends.