Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024	Dec. 31, 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss adjustment expense reserves, loss on delinquent payment	$ 0	$ 300,000	$ 0	$ 300,000
Loss and loss adjustment expenses	258,500,000	296,900,000	686,800,000	666,600,000
Other Operations
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	14,700,000		14,700,000			$ 12,100,000
Loss and loss adjustment expenses	600,000	4,200,000	18,800,000	8,100,000
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year losses	(100,000)	(24,800,000)	(70,000,000.0)	(32,000,000.0)
Loss and loss adjustment expense reserves	2,477,900,000	2,133,000,000	2,477,900,000	2,133,000,000	$ 2,288,500,000	2,127,500,000	$ 1,890,100,000	$ 1,605,100,000
Loss and loss adjustment expenses	254,600,000	288,300,000	652,100,000	644,000,000.0
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	2,477,900,000		2,477,900,000			2,127,500,000
Loss and loss adjustment expenses	254,600,000	288,300,000	652,100,000	644,000,000.0
P&C Insurance Distribution (Bamboo)
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expenses	3,300,000	4,400,000	15,900,000	14,500,000
P&C Insurance Distribution (Bamboo) | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	28,500,000		28,500,000			$ 17,800,000
Loss and loss adjustment expenses	$ 3,300,000	$ 4,400,000	$ 15,900,000	$ 14,500,000