Reserves for Losses and Loss Settlement Expenses - Reconciliation of the Disclosure Claims Development to the Liability for Claims and Claim Adjustment Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	$ 1,500,539
Net outstanding liabilities for unpaid unallocated loss settlement expenses	97,960
Fair value adjustment (purchase accounting adjustment for Mercer acquisition)	200
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	1,598,699
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	198,275
Reinsurance fair value amortization (purchase accounting adjustment for Mercer acquisition)	(192)
Total reinsurance recoverable on unpaid losses and loss settlement expenses	198,083
Total gross liability for unpaid losses and loss settlement expenses	1,796,782	$ 1,638,755	$ 1,497,274	$ 1,514,265
Commercial other liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	765,115
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	91,561
Commercial fire and allied
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	135,251
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	5,706
Commercial automobile
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	253,119
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	57
Commercial workers' compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	74,918
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	30,124
Personal
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	10,165
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	72
All other lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	261,971
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	$ 70,755