Schedule VI - Supplemental Information Concerning Property-Casualty Insurance Operations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Supplemental Information for Property, Casualty Insurance Underwriters
Current Year Loss and Loss Adjustment Expenses		$ 2,886.8	$ 2,711.1	$ 2,610.4
Prior Year Loss and Loss Adjustment Expenses		(305.8)	(282.6)	(210.6)
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity [Member]
Supplemental Information for Property, Casualty Insurance Underwriters
Deferred Policy Acquisition Costs	[1]	417.6	382.2	349.9
Loss and Loss Adjustment Expense Reserves	[1],[2]	6,955.2	6,824.8	6,587.0
Discount If Any Deducted From Reserves	[1]	179.9	184.7	174.8
Unearned Premiums	[1],[2]	2,253.1	2,028.5	1,870.7
Earned Premiums	[1]	4,119.2	3,808.6	3,555.5
Net Investment Income	[1]	462.7	358.0	342.4
Current Year Loss and Loss Adjustment Expenses	[1]	2,770.7	2,545.1	2,418.3
Prior Year Loss and Loss Adjustment Expenses	[1]	(234.0)	(193.1)	(137.9)
Amortization of Deferred Policy Acquisition Costs	[1]	680.0	591.2	501.9
Paid Loss and Loss Adjustment Expenses	[1]	2,406.2	2,114.2	2,021.3
Premiums Written	[1]	$ 4,356.3	$ 3,978.2	$ 3,680.9

[1] Includes consolidated property-casualty entities. The amounts relating to the Company's unconsolidated property-casualty subsidiaries and the proportionate share of the registrant's and its subsidiaries' 50%-or-less owned property-casualty equity investees are immaterial and have, therefore, been omitted from this schedule.

[2] See note (c) to Schedule III.