EMPLOYEE BENEFIT PLANS - Postretirement Benefits Other Than Pensions, Actuarial Gains and Losses, and Assumptions (Details) - Postretirement Medical Benefit Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Net unrecognized actuarial gains and losses related to the postretirement medical benefit obligation are included in accumulated other comprehensive income
Actuarial gain included in accumulated other comprehensive income at beginning of year	$ 460	$ (1,082)
Reclassification adjustment included in income	38	14
Plan amendments - prior service cost		896
Amortization of prior period cost	79
Actuarial gain (loss) per actuarial study	(821)	632
Actuarial gain (loss) included in accumulated other comprehensive income at end of year	$ (402)	$ 460
Assumptions used
Weighted-average discount rate	3.77%
Initial health care cost trend (2015)	7.65%
Ultimate health care cost trend	4.50%
Year that the rate reached the ultimate health care cost trend rate	2025
Average remaining service lives of active plan participants (years)	12 years 5 months 12 days
Impact of changes in the assumed health care cost trend rates upon the accumulated postretirement medical benefit obligation and the net periodic postretirement medical benefit expense
1% increase, Accumulated postretirement medical benefit obligation	$ 814
1% decrease, Accumulated postretirement medical benefit obligation	(708)
1% increase, Net periodic postretirement medical benefit expense	125
1% decrease, Net periodic postretirement medical benefit expense	$ (105)