Loss and Loss Adjustment Expense Reserves - Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2021	Dec. 31, 2020
Loss and loss adjustment expenses incurred relating to:
Net incurred loss and loss adjustment expenses	$ 185.8	$ 147.8
Syndicate - Ark Insurance Holdings Limited
Loss and loss adjustment expenses paid relating to:
Percent of underwriting capital provided by third parties			0.00%	42.80%
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,605.1	1,296.5
Less: beginning reinsurance recoverable on unpaid losses	(340.8)	(505.0)
Net loss and loss adjustment expense reserves	1,264.3	791.5
Loss and loss adjustment expenses incurred relating to:
Current year losses	180.3	139.2
Prior year losses	(0.3)	8.6
Net incurred loss and loss adjustment expenses	180.0	147.8
Loss and loss adjustment expenses paid relating to:
Current year losses	(4.0)	(4.0)
Prior year losses	(96.7)	(87.7)
Net paid loss and loss adjustment expenses	(100.7)	(91.7)
Change in TPC providers’ participation	0.0	145.4
Foreign currency translation and other adjustments to loss and loss adjustment expense reserves	(3.6)	3.7
Net ending balance	1,340.0	996.7
Plus: ending reinsurance recoverable on unpaid losses	(404.5)	(348.9)
Gross ending balance	1,744.5	1,345.6
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,605.1
Loss and loss adjustment expenses incurred relating to:
Net incurred loss and loss adjustment expenses	180.0	147.8
Loss and loss adjustment expenses paid relating to:
Gross ending balance	1,744.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | TPC Providers
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: beginning reinsurance recoverable on unpaid losses		(145.4)
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	0.0
Loss and loss adjustment expenses incurred relating to:
Net incurred loss and loss adjustment expenses	5.8	$ 0.0
Loss and loss adjustment expenses paid relating to:
Gross ending balance	$ 12.3