PENSION AND OTHER POST-EMPLOYMENT BENEFITS - Reconciliation of Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets:
Fair value of plan assets at beginning of the period	$ 263,027
Fair value of plan assets at the end of period	273,895	$ 263,027
Pension Benefits
Change in projected benefit obligation:
Projected benefit obligation at beginning of the period	280,193	273,809
Service cost	700	1,870	$ 7,457
Interest cost	9,455	9,662	12,350
Plan amendments	0	0
Participant contributions	0	0
Actuarial (gain) loss	27,925	7,207
Benefits paid	(19,586)	(11,943)
(Gain) related to settlement	(472)	0
Administrative expenses paid	(2,649)	(412)
Projected benefit obligation at end of the period	295,566	280,193	273,809
Accumulated benefit obligation at end of the period	295,566	280,193
Change in plan assets:
Fair value of plan assets at beginning of the period	263,027	210,556
Actual return on plan assets	33,103	11,662
Employer contributions	0	53,164
Participant contributions	0	0
Actual expenses paid	(2,649)	(412)
Benefits paid	(19,586)	(11,943)
Fair value of plan assets at the end of period	273,895	263,027	210,556
Funded status	(21,671)	(17,166)
Other Benefits
Change in projected benefit obligation:
Projected benefit obligation at beginning of the period	5,736	6,294
Service cost	0	0	5
Interest cost	173	196	289
Plan amendments	(1,317)	(998)
Participant contributions	206	206
Actuarial (gain) loss	(284)	1,076
Benefits paid	(708)	(1,038)
(Gain) related to settlement	0	0
Administrative expenses paid	0	0
Projected benefit obligation at end of the period	3,806	5,736	6,294
Accumulated benefit obligation at end of the period	0	0
Change in plan assets:
Fair value of plan assets at beginning of the period	0	0
Actual return on plan assets	0	0
Employer contributions	502	832
Participant contributions	206	206
Benefits paid	(708)	(1,038)
Fair value of plan assets at the end of period	0	0	$ 0
Funded status	$ (3,806)	$ (5,736)