PENSION AND OTHER POST-EMPLOYMENT BENEFITS - Components of Net Periodic Benefit Cost (Income) Pension and OBEB Plans (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of the net periodic benefit cost
Settlement related expense	$ 500	$ 600	$ 4,600	$ 2,200
Net periodic benefit cost					$ (7,107)	$ (9,594)	$ (5,800)
Pension Benefits
Components of the net periodic benefit cost
Expected administrative expenses					940	700	1,870
Interest cost					10,160	9,455	9,662
Expected return on plan assets					(17,574)	(18,444)	(14,782)
Amortization of prior service cost					0	0	0
Recognized actuarial loss					988	704	492
Settlement related expense					5,735	2,162	0
Net periodic benefit cost					249	(5,423)	(2,758)
Other Benefits
Components of the net periodic benefit cost
Expected administrative expenses					0	0	0
Interest cost					120	173	196
Expected return on plan assets					0	0	0
Amortization of prior service cost					(730)	(1,485)	(1,120)
Recognized actuarial loss					(71)	3	(248)
Settlement related expense					0	0	0
Net periodic benefit cost					$ (681)	$ (1,309)	$ (1,172)