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, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	$ 1,358,808
Net outstanding liabilities for unpaid unallocated loss settlement expenses	87,949
Fair value adjustment (purchase accounting adjustment for Mercer acquisition)	560
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	1,447,317
Reinsurance Recoverables on Unpaid Losses, Gross	191,974
Reinsurance fair value amortization (purchase accounting adjustment for Mercer acquisition)	(536)
Total reinsurance recoverable on unpaid losses and loss settlement expenses	191,438
Total gross liability for unpaid losses and loss settlement expenses	1,638,755	$ 1,497,274	$ 1,514,265	$ 1,578,131
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	678,815	$ 678,815
Reinsurance Recoverables on Unpaid Losses, Gross	85,753
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	147,570
Reinsurance Recoverables on Unpaid Losses, Gross	9,750
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	281,722
Reinsurance Recoverables on Unpaid Losses, Gross	2,624
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	73,014
Reinsurance Recoverables on Unpaid Losses, Gross	38,231
Personal
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	7,568
Reinsurance Recoverables on Unpaid Losses, Gross	294
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	170,119
Reinsurance Recoverables on Unpaid Losses, Gross	$ 55,322