EMPLOYEE BENEFIT PLANS - Postretirement (Details) - Postretirement benefit plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postretirement benefit plan disclosure
Assumed ultimate health care cost trend rate (as a percent)	3.00%
Accumulated benefit obligation after plan amendment	$ 13,497	$ 12,128
Amounts recognized in accumulated other comprehensive loss (pre-tax)
Prior service credit	(5,519)
Net actuarial gain	(713)
Net amount recognized in accumulated other comprehensive loss	(6,232)
Estimated amount to be amortized from accumulated other comprehensive loss (gain) into net periodic benefit cost during next fiscal year
Actuarial gain	97
Prior service credit	1,226
Changes in the accumulated postretirement benefit obligation
Benefit obligation, beginning of the period	12,128	11,400
Service cost	323	331	$ 441
Interest cost	468	462	465
Actuarial (gain)/loss	897	235
Benefits paid	(319)	(300)
Benefit obligation, end of the period	13,497	12,128	11,400
Net periodic postretirement benefit cost
Service cost benefits attributed to service during the period	323	331	441
Interest cost on the accumulated postretirement benefit obligation	468	462	465
Net amortization	(1,462)	(1,642)	(1,451)
Net periodic postretirement benefit cost (income)	$ (671)	$ (849)	$ (545)