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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net outstanding liabilities	$ 1,574,352	$ 1,461,348	$ 1,271,503	$ 1,139,337
Unallocated loss adjustment expenses	52,275	50,891
Other	13,237	9,998
Liabilities for losses and loss adjustment expenses, net of reinsurance	1,189,835	1,096,349
Allowance for uncollectible reinsurance balances recoverable on unpaid losses and settlement expenses	9,402	9,793
Other	982	980
Reinsurance recoverable on unpaid claims	384,517	364,999	$ 301,991	$ 288,224
Total gross liability for unpaid loss and settlement expenses	1,574,352	1,461,348
Casualty segment | Primary Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	403,910	372,450
Reinsurance recoverable on unpaid claims	31,122	34,742
Casualty segment | Excess Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	256,153	209,683
Reinsurance recoverable on unpaid claims	98,518	81,072
Casualty segment | Claims Made
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	217,954	204,501
Reinsurance recoverable on unpaid claims	176,936	144,921
Casualty segment | Transportation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	139,951	133,558
Reinsurance recoverable on unpaid claims	53,724	50,748
Property segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	71,965	77,238
Reinsurance recoverable on unpaid claims	21,438	50,495
Surety segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Liabilities for losses and loss adjustment expenses, net of reinsurance	24,988	28,237
Reinsurance recoverable on unpaid claims	$ 11,199	$ 11,834