PENSION AND OTHER POST-EMPLOYMENT BENEFITS - Balance Sheet Components (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Amounts recognized in the consolidated balance sheets consist of:
Current liabilities	$ 0	$ 0
Noncurrent liabilities	(21,671)	(17,166)
Net amount recognized	(21,671)	(17,166)
Amounts recognized in accumulated other comprehensive income (loss) before taxes:
Net actuarial loss (gain)	60,256	50,327
Prior service cost (credit)	0	0
Net amount recognized	60,256	50,327
Other Benefits
Amounts recognized in the consolidated balance sheets consist of:
Current liabilities	(231)	(612)
Noncurrent liabilities	(3,575)	(5,124)
Net amount recognized	(3,806)	(5,736)
Amounts recognized in accumulated other comprehensive income (loss) before taxes:
Net actuarial loss (gain)	1,015	1,302
Prior service cost (credit)	(3,310)	(3,477)
Net amount recognized	$ (2,295)	$ (2,175)