Reserve for Loss and Loss Expense - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred loss and loss expense for claims occurring in the prior year	$ (22,545)	$ 70,096
Casualty lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred loss and loss expense for claims occurring in the prior year		90,000
Casualty lines \| Standard Commercial Lines \| Commercial automobile
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred loss and loss expense for claims occurring in the prior year		60,000
Casualty lines \| Standard Commercial Lines \| General liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred loss and loss expense for claims occurring in the prior year		20,000
Casualty lines \| Standard Personal Lines \| Personal automobile
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred loss and loss expense for claims occurring in the prior year		10,000
Property lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred loss and loss expense for claims occurring in the prior year		$ (19,900)