Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	11 Months Ended	12 Months Ended
	Dec. 05, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss adjustment expense reserves, loss on delinquent payment			$ 0.3
Loss and loss adjustment expenses		$ 882.2	888.5	$ 726.8
Other Operations
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves		13.6	12.1
Loss and loss adjustment expenses		20.0	12.1	0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Change in TPC providers’ participation		0.0	0.0	145.4
Favorable prior loss reserve development		(112.7)	(58.6)	19.9
Loss and loss adjustment expense reserves		2,481.0	2,127.5	1,605.1	$ 1,296.5
Loss and loss adjustment expenses		844.2	855.8	726.8
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		2,481.0	2,127.5
Loss and loss adjustment expenses		844.2	855.8	726.8
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves			17.8
Loss and loss adjustment expenses	$ 18.0	18.0	20.6	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%
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		(91.7)	29.2	(41.7)
Property and Accident & Health | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Aviation Losses from Ukraine and Russia Conflict
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		91.4
Specialty | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		(27.2)	34.3	(11.9)
Marine & Energy | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		4.7
Casualty-Active | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		$ 4.3	$ 4.5
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)