Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Net incurred loss and loss adjustment expenses	$ 185.8	$ 147.8
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%
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year losses	(0.3)	8.6
Loss and loss adjustment expense reserves	1,744.5	1,345.6	$ 1,605.1	$ 1,296.5
Net incurred loss and loss adjustment expenses	180.0	147.8
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	1,744.5		1,605.1
Net incurred loss and loss adjustment expenses	180.0	147.8
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	12.3		$ 0.0
Net incurred loss and loss adjustment expenses	$ 5.8	$ 0.0