Loss and Loss Adjustment Expense Reserves - LAE Reserve Activity (Details) - Group Ark Insurance Limited	12 Months Ended
	Dec. 31, 2021 USD ($)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	$ 696,000,000.0
Less: beginning reinsurance recoverable on unpaid losses	(433,400,000)
Net loss and LAE reserves	262,600,000
Losses and LAE incurred relating to:
Current Year	336,300,000
Prior Year	(21,500,000)
Net incurred losses and LAE	314,800,000
Loss and LAE paid relating to:
Current year losses	(43,900,000)
Prior year losses	(61,600,000)
Net paid losses and LAE	(105,500,000)
Foreign currency translation and other adjustments to loss and LAE reserves	(3,900,000)
Net ending balance	465,800,000
Plus: ending reinsurance recoverable on unpaid losses	(428,900,000)
Gross ending balance	894,700,000
Amount attributable to TPC Providers
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: beginning reinsurance recoverable on unpaid losses	319,200,000
Loss and LAE paid relating to:
Change in TPC Providers’ participation	(2,200,000)
Plus: ending reinsurance recoverable on unpaid losses	$ (276,800,000)