Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	$ 630.0	$ 591.7
Capitalizations	314.1	281.0
Amortization Expense	(284.3)	(264.4)
Ending balance	659.8	608.3
Variable Interest Entity, Primary Beneficiary
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	1.1	0.3
Ending balance	1.6	0.1
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	625.9	569.7
Capitalizations	310.9	272.3
Amortization Expense	(280.1)	(243.0)
Ending balance	656.7	599.0
Operating Segments | Specialty Property & Casualty Insurance
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	162.8	142.7
Capitalizations	278.3	236.6
Amortization Expense	(266.0)	(227.3)
Ending balance	175.1	152.0
Operating Segments | Life Insurance
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	463.1	427.0
Capitalizations	32.6	35.7
Amortization Expense	(14.1)	(15.7)
Ending balance	481.6	447.0
Corporate and Other
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	4.1	22.0
Capitalizations	3.2	8.7
Amortization Expense	(4.2)	(21.4)
Ending balance	$ 3.1	$ 9.3