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, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	$ 1,308,438
Net outstanding liabilities for unpaid unallocated loss settlement expenses	92,084
Fair value adjustment (purchase accounting adjustment for Mercer acquisition)	843
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	1,401,365
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	113,706
Reinsurance fair value amortization (purchase accounting adjustment for Mercer acquisition)	(806)
Total reinsurance recoverable on unpaid losses and loss settlement expenses	112,900
Total gross liability for unpaid losses and loss settlement expenses	1,514,265	$ 1,578,131	$ 1,421,754	$ 1,312,483
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	612,108
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	22,522
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	122,739
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	21,309
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	383,149
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	4,819
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	103,856
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	53,189
Personal
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid losses and allocated loss settlement expenses	22,503
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	3,236
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	64,083
Reinsurance recoverable on unpaid losses and allocated loss settlement expenses	$ 8,631