HISTORICAL LOSS AND LAE DEVELOPMENT - Reconciliation of Incurred and Paid Loss Develeopment to the Liability for Unpaid Losses and Settlement Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	$ 1,271,503	$ 1,139,337	$ 1,103,785	$ 1,121,040
Unallocated loss adjustment expenses	48,844	46,286
Other	13,322	14,279
Liabilities for losses and loss adjustment expenses, net of reinsurance	969,512	851,113
Allowance for uncollectible reinsurance balances recoverable on unpaid losses and settlement expenses	10,014	10,699
Other	1,729	2,185
Reinsurance recoverable on unpaid claims	301,991	288,224	$ 297,844	$ 335,106
Total gross liability for unpaid loss and settlement expenses	1,271,503	1,139,337
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	325,182	293,256
Reinsurance recoverable on unpaid claims	36,158	43,662
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	188,963	163,801
Reinsurance recoverable on unpaid claims	74,400	69,858
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	171,241	152,031
Reinsurance recoverable on unpaid claims	117,436	113,891
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	119,704	98,604
Reinsurance recoverable on unpaid claims	46,590	41,173
Property segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	68,867	47,769
Reinsurance recoverable on unpaid claims	28,613	17,548
Surety segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	23,375	24,388
Reinsurance recoverable on unpaid claims	$ 7,079	$ 10,606