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, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	$ 1,261,587
Net outstanding liabilities for unpaid unallocated loss settlement expenses	88,252
Fair value adjustment (purchase accounting adjustment for Mercer acquisition)	560
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	1,350,399
Reinsurance Recoverables on Unpaid Losses, Gross	147,411
Reinsurance fair value amortization (purchase accounting adjustment for Mercer acquisition)	(536)
Total reinsurance recoverable on unpaid losses and loss settlement expenses	146,875
Total gross liability for unpaid losses and loss settlement expenses	1,497,274	$ 1,514,265	$ 1,578,131	$ 1,421,754
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	646,054	646,054
Reinsurance Recoverables on Unpaid Losses, Gross	55,381
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	126,432	126,432
Reinsurance Recoverables on Unpaid Losses, Gross	19,423
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	299,668	299,668
Reinsurance Recoverables on Unpaid Losses, Gross	2,607
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	84,019	84,019
Reinsurance Recoverables on Unpaid Losses, Gross	44,588
Personal
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	10,339	$ 10,339
Reinsurance Recoverables on Unpaid Losses, Gross	918
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	95,075
Reinsurance Recoverables on Unpaid Losses, Gross	$ 24,494