Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	$ 630.0	$ 591.7
Capitalizations	159.6	136.6
Amortization Expense	(138.4)	(132.4)
Ending balance	651.2	595.9
Experience adjustment	0.9	1.4
Variable Interest Entity, Primary Beneficiary
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	1.1	0.6
Ending balance	1.5	0.1
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	625.9	569.7
Capitalizations	157.9	129.7
Amortization Expense	(136.0)	(119.7)
Ending balance	647.8	579.7
Operating Segments | Specialty Property & Casualty Insurance
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	162.8	142.7
Capitalizations	143.0	112.9
Amortization Expense	(128.9)	(112.0)
Ending balance	176.9	143.6
Operating Segments | Life Insurance
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	463.1	427.0
Capitalizations	14.9	16.8
Amortization Expense	(7.1)	(7.7)
Ending balance	470.9	436.1
Corporate and Other
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning balance	4.1	22.0
Capitalizations	1.7	6.9
Amortization Expense	(2.4)	(12.7)
Ending balance	$ 3.4	$ 16.2