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, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Unpaid losses and settlement expenses	$ 2,315,637	$ 2,043,555	$ 1,750,049	$ 1,574,352
Liabilities for losses and loss adjustment expenses, net of reinsurance	1,575,548	1,435,469
Unallocated loss adjustment expenses	61,752	59,391
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	11,250	11,188
Other	30,377	27,037
Reinsurance recoverable on unpaid claims	740,089	608,086	$ 443,729	$ 384,517
Other	107	111
Total gross liability for unpaid loss and settlement expenses	2,315,637	2,043,555
Casualty Segment | Primary Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	461,527	453,142
Reinsurance recoverable on unpaid claims	46,674	43,688
Casualty Segment | Excess Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	467,592	384,579
Reinsurance recoverable on unpaid claims	138,822	122,092
Casualty Segment | Claims Made
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	211,296	209,705
Reinsurance recoverable on unpaid claims	298,930	271,259
Casualty Segment | Transportation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	141,737	131,531
Reinsurance recoverable on unpaid claims	78,456	66,283
Property segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	149,901	124,652
Reinsurance recoverable on unpaid claims	137,613	65,098
Surety Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	40,116	34,244
Reinsurance recoverable on unpaid claims	$ 50,737	$ 50,743