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, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Unpaid losses and settlement expenses	$ 2,446,025	$ 2,315,637	$ 2,043,555	$ 1,750,049
Liabilities for losses and loss adjustment expenses, net of reinsurance	1,688,676	1,575,548
Unallocated loss adjustment expenses	66,111	61,752
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	10,608	11,250
Other	26,931	30,377
Reinsurance recoverable on unpaid claims	757,349	740,089	$ 608,086	$ 443,729
Other	109	107
Total gross liability for unpaid loss and settlement expenses	2,446,025	2,315,637
Casualty Segment | Primary Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	482,569	461,527
Reinsurance recoverable on unpaid claims	53,177	46,674
Casualty Segment | Excess Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	556,978	467,592
Reinsurance recoverable on unpaid claims	149,502	138,822
Casualty Segment | Claims Made
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	207,564	211,296
Reinsurance recoverable on unpaid claims	292,803	298,930
Casualty Segment | Transportation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	148,226	141,737
Reinsurance recoverable on unpaid claims	68,126	78,456
Property segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	142,876	149,901
Reinsurance recoverable on unpaid claims	152,714	137,613
Surety Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	46,813	40,116
Reinsurance recoverable on unpaid claims	$ 51,526	$ 50,737