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, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	$ 1,139,337	$ 1,103,785	$ 1,121,040	$ 1,129,433
Unallocated loss adjustment expenses	46,286	42,222
Other	14,279	20,024
Liabilities for losses and loss adjustment expenses, net of reinsurance	851,113	805,941
Allowance for uncollectible reinsurance balances recoverable on unpaid losses and settlement expenses	10,699	11,885
Other	2,185	2,691
Total reinsurance balances recoverable on unpaid losses and settlement expenses	288,224	297,844	$ 335,106	$ 354,924
Total gross liability for unpaid loss and settlement expenses	1,139,337	1,103,785
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	293,256	292,906
Total reinsurance balances recoverable on unpaid losses and settlement expenses	43,662	47,186
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	163,801	151,535
Total reinsurance balances recoverable on unpaid losses and settlement expenses	69,858	80,813
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	152,031	145,778
Total reinsurance balances recoverable on unpaid losses and settlement expenses	113,891	130,701
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	98,604	74,750
Total reinsurance balances recoverable on unpaid losses and settlement expenses	41,173	25,006
Property segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	47,769	46,542
Total reinsurance balances recoverable on unpaid losses and settlement expenses	17,548	14,469
Surety segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	24,388	20,299
Total reinsurance balances recoverable on unpaid losses and settlement expenses	$ 10,606	$ 8,863