Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2024	Dec. 31, 2023	Mar. 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	$ 183.9	$ 167.5	$ 369.7	$ 315.3
Other Operations
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	3.9		3.9			$ 0.0
Net incurred loss and loss adjustment expenses	3.9	0.0	3.9	0.0
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	(6.9)	11.0	(7.2)	19.6
Loss and loss adjustment expense reserves	1,890.1	1,421.0	1,890.1	1,421.0	$ 1,744.5	1,605.1	$ 1,345.6	$ 1,296.5
Net incurred loss and loss adjustment expenses	175.7	167.5	355.7	315.3
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	1,890.1		1,890.1			1,605.1
Net incurred loss and loss adjustment expenses	175.7	167.5	355.7	315.3
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	16.6		16.6			$ 0.0
Net incurred loss and loss adjustment expenses	$ 4.3	$ 0.0	$ 10.1	$ 0.0