Reserve for Loss and Loss Expense - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
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			$ 187,121	$ (8,067)
Casualty reserve
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	$ 176,000	$ (3,500)	211,000	(16,500)
Casualty reserve | Workers' compensation
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			(15,000)	(17,500)
Casualty reserve | E&S
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				(5,000)
Casualty reserve | 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			5,000	6,000
Casualty reserve | 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			216,000
Casualty reserve | 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			10,000
Casualty reserve | Standard Personal Lines | Homeowners
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			(5,000)
Property reserve
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			$ (23,900)	$ 8,400