Historical Loss and LAE Development - Reconciliation of Incurred and Paid Loss Development to the Liability for Unpaid Losses and Settlement 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
Unpaid loss and settlement expenses	$ 2,693,470	$ 2,446,025	$ 2,315,637	$ 2,043,555
Liabilities for losses and loss adjustment expenses, net of reinsurance	1,938,045	1,688,676
Unallocated loss adjustment expenses	73,613	66,111
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	9,580	10,608
Other	22,940	26,931
Reinsurance recoverable on unpaid claims	755,425	757,349	$ 740,089	$ 608,086
Other	132	109
Casualty Segment | Primary Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	511,759	482,569
Reinsurance recoverable on unpaid claims	47,773	53,177
Casualty Segment | Excess Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	656,982	556,978
Reinsurance recoverable on unpaid claims	164,721	149,502
Casualty Segment | Claims Made
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	199,979	207,564
Reinsurance recoverable on unpaid claims	283,560	292,803
Casualty Segment | Transportation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	185,023	148,226
Reinsurance recoverable on unpaid claims	76,089	68,126
Property segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	228,230	142,876
Reinsurance recoverable on unpaid claims	102,120	152,714
Surety Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	49,939	46,813
Reinsurance recoverable on unpaid claims	$ 90,610	$ 51,526