EMPLOYEE BENEFIT PLANS Table 1 - Changes in Benefit Obligation/Fair Value of Plan Assets/Funded Status/Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Change in benefit obligation:
Benefit obligation, January 1	$ 127,402	$ 109,202
Service cost	8,955	7,703	$ 7,676
Interest cost	4,507	4,023	4,389
Benefits paid	(5,941)	(2,554)
Participant contributions	0	0
Actuarial loss (gain)	14,894	9,028
Benefit obligation, December 31	149,817	127,402	109,202
Change in plan assets:
Plan assets at fair value, January 1	107,644	87,706
Actual return on plan assets	17,070	6,891
Employer contributions	11,105	15,601
Benefits paid	(5,941)	(2,554)
Participant contributions	0	0
Plan assets at fair value, December 31	129,878	107,644	87,706
Reconciliation of funded status:
Fair value of plan assets at December 31	107,644	87,706	87,706	$ 129,878	$ 107,644
Less: Benefit obligation at December 31	127,402	109,202	109,202	149,817	127,402
Funded status at December 31				(19,939)	(19,758)
Amounts recognized in the consolidated balance sheets:
Accrued liabilities				(210)	(162)
Other long-term liabilities				(19,729)	(19,596)
Net pension liability				(19,939)	(19,758)
Other Postretirement Benefits Plan [Member]
Change in benefit obligation:
Benefit obligation, January 1	11,061	10,042
Service cost	456	419	470
Interest cost	430	401	448
Benefits paid	(342)	(422)
Participant contributions	215	253
Actuarial loss (gain)	590	368
Benefit obligation, December 31	12,410	11,061	10,042
Change in plan assets:
Plan assets at fair value, January 1	0	0
Actual return on plan assets	0	0
Employer contributions	127	169
Benefits paid	(342)	(422)
Participant contributions	215	253
Plan assets at fair value, December 31	0	0	0
Reconciliation of funded status:
Fair value of plan assets at December 31	0	0	0	0	0
Less: Benefit obligation at December 31	$ 11,061	$ 10,042	$ 10,042	12,410	11,061
Funded status at December 31				(12,410)	(11,061)
Amounts recognized in the consolidated balance sheets:
Accrued liabilities				(376)	(321)
Other long-term liabilities				(12,034)	(10,740)
Net pension liability				$ (12,410)	$ (11,061)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

Name:	us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.