Related Party Transactions - Schedule of Commissions (Details) - Related Party - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Related Party Transaction [Line Items]
Ceding commission expense	$ 89.2	$ 87.1	$ 244.2	$ 236.1
Profit commission expense	7.6	0.0	9.9	0.0
The Fidelis Partnership commissions	$ 96.8	$ 87.1	$ 254.1	$ 236.1

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details