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, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Unpaid losses and settlement expenses	$ 2,043,555	$ 1,750,049	$ 1,574,352	$ 1,461,348
Liabilities for losses and loss adjustment expenses, net of reinsurance	1,435,469	1,306,320
Unallocated loss adjustment expenses	59,391	54,954
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	11,188	8,634
Other	27,037	22,735
Reinsurance recoverable on unpaid claims	608,086	443,729	$ 384,517	$ 364,999
Other	111	8,088
Total gross liability for unpaid loss and settlement expenses	2,043,555	1,750,049
Casualty Segment | Primary Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	453,142	432,823
Reinsurance recoverable on unpaid claims	43,688	35,202
Casualty Segment | Excess Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	384,579	312,827
Reinsurance recoverable on unpaid claims	122,092	88,528
Casualty Segment | Claims Made
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	209,705	214,103
Reinsurance recoverable on unpaid claims	271,259	223,020
Casualty Segment | Transportation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	131,531	129,852
Reinsurance recoverable on unpaid claims	66,283	53,251
Property segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	124,652	103,383
Reinsurance recoverable on unpaid claims	65,098	40,257
Surety Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	34,244	27,009
Reinsurance recoverable on unpaid claims	$ 50,743	$ 4,017