Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ 19.9	$ (51.7)	$ (21.5)
Syndicate - Ark Insurance Holdings Limited
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of underwriting capital provided by third parties	0.00%	0.00%	0.00%	42.80%	58.30%	57.60%	60.00%	59.60%	70.00%	66.20%
Property and Accident & Health \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ 41.7	$ (20.8)	$ 8.9
Specialty \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(11.9)	(12.7)	3.3
Casualty-Runoff \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ (6.0)		3.3
Marine & Energy \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		$ (18.8)
Casualty-Active \| Ark
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development			$ 3.7