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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	$ 1,461,348	$ 1,271,503	$ 1,139,337	$ 1,103,785
Unallocated loss adjustment expenses	50,891	48,844
Other	9,998	13,322
Liabilities for losses and loss adjustment expenses, net of reinsurance	1,096,349	969,512
Allowance for uncollectible reinsurance balances recoverable on unpaid losses and settlement expenses	9,793	10,014
Other	980	1,729
Reinsurance recoverable on unpaid claims	364,999	301,991	$ 288,224	$ 297,844
Total gross liability for unpaid loss and settlement expenses	1,461,348	1,271,503
Casualty segment | Primary Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	372,450	325,182
Reinsurance recoverable on unpaid claims	34,742	36,158
Casualty segment | Excess Occurrence
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	209,683	188,963
Reinsurance recoverable on unpaid claims	81,072	74,400
Casualty segment | Claims Made
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	204,501	171,241
Reinsurance recoverable on unpaid claims	144,921	117,436
Casualty segment | Transportation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	133,558	119,704
Reinsurance recoverable on unpaid claims	50,748	46,590
Property segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	77,238	68,867
Reinsurance recoverable on unpaid claims	50,495	28,613
Surety segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	28,237	23,375
Reinsurance recoverable on unpaid claims	$ 11,834	$ 7,079