Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss adjustment expense reserves, loss on delinquent payment	$ 0.3		$ 0.3
Net incurred loss and loss adjustment expenses	296.9	$ 265.8	666.6	$ 581.1
Other Operations
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	8.1		8.1			$ 0.0
Net incurred loss and loss adjustment expenses	4.2	0.0	8.1	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	(24.8)	(0.6)	(32.0)	19.0
Loss and loss adjustment expense reserves	2,133.0	1,571.8	2,133.0	1,571.8	$ 1,890.1	1,605.1	$ 1,421.0	$ 1,296.5
Net incurred loss and loss adjustment expenses	288.3	265.8	644.0	581.1
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	2,133.0		2,133.0			1,605.1
Net incurred loss and loss adjustment expenses	288.3	265.8	644.0	581.1
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	17.7		17.7			$ 0.0
Net incurred loss and loss adjustment expenses	$ 4.4	$ 0.0	$ 14.5	$ 0.0