Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss adjustment expense reserves, loss on delinquent payment	$ 0.3
Loss and loss adjustment expenses	888.5	$ 726.8	$ 536.4
Other Operations
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	12.1	0.0
Loss and loss adjustment expenses	12.1	0.0	0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(58.6)	19.9	(51.7)
Loss and loss adjustment expense reserves	2,127.5	1,605.1	1,296.5	$ 894.7
Loss and loss adjustment expenses	855.8	726.8	536.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	2,127.5	1,605.1
Loss and loss adjustment expenses	855.8	726.8	536.4
P&C Insurance Distribution (Bamboo)
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expenses	20.6
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	17.8	0.0
Loss and loss adjustment expenses	20.6	0.0	0.0
Syndicate - Ark Insurance Holdings Limited
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of underwriting capital provided by third parties				0.00%	42.80%	58.30%
Property and Accident & Health | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(29.2)	41.7	(20.8)
Specialty | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(34.3)	11.9	(12.7)
Casualty-Runoff | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		$ 6.0
Marine & Energy | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development			$ (18.8)
Casualty-Active | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ 4.5