LIABILITY FOR LOSSES AND SETTLEMENT EXPENSES RECONCILIATION TO LIABILITY (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 liability	$ 716,495
Ceded reserves	36,595	$ 30,923	$ 20,664	$ 23,477
Unallocated settlement expenses	24,100
Gross reserve for losses and settlement expenses	777,190	$ 732,612	$ 690,532	$ 678,774
Commercial Auto Liability [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	120,398
Ceded reserves	841
Commercial Property [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	30,213
Ceded reserves	9,962
Workers' Compensation [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	144,749
Ceded reserves	12,956
Other Liability [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	174,076
Ceded reserves	3,830
Personal Auto Liability [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	5,632
Ceded reserves	1,274
Homeowners Insurance [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	1,722
Ceded reserves	275
Auto Physical Damage [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	1,590
Ceded reserves	103
Assumed Pro Rata [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	52,105
Ceded reserves	5,453
Assumed Excess of Loss [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	184,299
Ceded reserves	1,109
Other Short-duration Insurance Product Line [Member]
Short-duration insurance contracts, reconciliation of claims development to liability [Line Items]
Net outstanding liability	1,711
Ceded reserves	$ 792