Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	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	Dec. 31, 2013
Ark | Amount attributable to TPC Providers
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of underwriting capital provided by third parties	0.00%	0.00%	42.80%	58.30%	57.60%	60.00%	59.60%	70.00%	66.20%	0.00%
Group Ark Insurance Limited
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ (51.7)	$ (21.5)
Group Ark Insurance Limited | Property and Accident & Health
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(20.8)	8.9
Group Ark Insurance Limited | Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	(12.7)	3.3
Group Ark Insurance Limited | Marine & Energy
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development	$ (18.8)
Group Ark Insurance Limited | Casualty - Active
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		3.7
Group Ark Insurance Limited | Casualty - Runoff
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable prior loss reserve development		$ 3.3