LIABILITY FOR LOSSES AND SETTLEMENT EXPENSES RECONCILIATION TO LIABILITY (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 liability	$ 674,501
Ceded reserves	30,923	$ 20,664	$ 23,477	$ 28,253
Unallocated settlement expenses	27,188
Gross reserve for losses and settlement expenses	732,612	$ 690,532	$ 678,774	$ 661,309
Commercial Auto Liability [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	111,134
Ceded reserves	768
Commercial Property [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	30,542
Ceded reserves	4,624
Workers' Compensation [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	138,322
Ceded reserves	10,005
Other Liability [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	163,679
Ceded reserves	3,938
Personal Auto Liability [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	6,711
Ceded reserves	1,337
Homeowners Insurance [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	2,231
Ceded reserves	573
Auto Physical Damage [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	654
Ceded reserves	228
Assumed Pro Rata [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	55,639
Ceded reserves	7,167
Assumed Excess of Loss [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	163,306
Ceded reserves	1,444
Other Short-duration Insurance Product Line [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	2,283
Ceded reserves	$ 839