PENSION AND OTHER POST-EMPLOYMENT BENEFITS - Components of Net Periodic Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Components of the net periodic benefit cost
Service cost	$ 700	$ 1,870	$ 7,457
Interest cost	9,455	9,662	12,350
Expected return on plan assets	(18,444)	(14,782)	(14,455)
Amortization of prior service cost	0	0	0
Recognized actuarial loss	704	492	6,311
Settlement and curtailment related expense	2,162	0	0
Net periodic benefit cost	(5,423)	(2,758)	11,663
Other Benefits
Components of the net periodic benefit cost
Service cost	0	0	5
Interest cost	173	196	289
Expected return on plan assets	0	0	0
Amortization of prior service cost	(1,485)	(1,120)	(852)
Recognized actuarial loss	3	(248)	(144)
Settlement and curtailment related expense	0	0	0
Net periodic benefit cost	$ (1,309)	$ (1,172)	$ (702)