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EMPLOYEE BENEFITS - Postretirement Benefits (Details) - Postretirement Benefits [Member] - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Postretirement Benefits      
Service cost $ 8,238 $ 10,128 $ 9,005
Interest cost 9,855 9,649 10,549
Expected return on assets (10,113) (10,375) (8,237)
Amortization of prior service credits (6,688) (6,801) (7,254)
Amortization of transition asset 0 0 (143)
Amortization of unrecognized losses 129 1,512 779
Net periodic benefit costs 1,421 4,113 4,699
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 239,348 282,917  
Service cost 8,238 10,128 9,005
Interest cost 9,855 9,649 10,549
Plan participants' contributions 2,943 2,754  
Actuarial losses (gains) 13,218 (58,251)  
Benefits paid (9,439) (8,739)  
Prescription drug rebates 865 890  
Benefit obligation at end of year 265,028 239,348 $ 282,917
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Plan participants' contributions 2,943 2,754  
Benefits paid (9,439) (8,739)  
Prescription drug rebates 865 890  
Noncurrent postretirement benefit obligation 101,483 83,737  
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]      
Prior service credit 53,814 60,502  
Unrecognized losses (12,656) (3,015)  
Deferred tax (liability) (15,861) (22,134)  
Net accumulated gains 25,297 $ 35,353  
Increase in unrecognized Losses in AOCE 10,000    
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year [Abstract]      
Amortization of prior service credit (6,492)    
Amortization of unrecognized losses 937    
Estimated amount to be amortized from AOCE into net periodic postretirement benefit costs $ (5,555)    
Net unrecognized gains (losses), amortization period 13 years 6 months    
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]      
Discount rate 4.20% 3.89% 4.90%
Long-term rate of return on plan assets, net of tax 6.65% 6.65% 5.70%
Initial healthcare cost trend rate 7.00% 7.25% 7.50%
Ultimate healthcare cost trend rate 4.50% 4.50% 4.50%
Year ultimate healthcare cost trend rate reached 2026 2026 2026
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]      
Discount rate 4.00% 4.20% 3.89%
Expected long-term rate of return on plan assets, net of tax 7.13% 6.65% 6.65%
Initial healthcare cost trend rate 6.81% 7.00% 7.25%
Ultimate healthcare cost trend rate 4.50% 4.50% 4.50%
Year ultimate healthcare cost trend rate reached 2026 2026 2026
Initial healthcare cost trend rate, post age 65 9.36%    
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]      
Effect of one percentage point increase on service and interest cost $ 1,411    
Effect of one percentage point decrease on service and interest cost (1,162)    
Effect of one percentage point increase on postretirement benefit obligation 27,542    
Effect of one percentage point decrease on postretirement benefit obligation (22,748)    
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]      
Estimated gross benefit payments, 2017 8,211    
Estimated gross benefit payments, 2018 9,236    
Estimated gross benefit payments, 2019 10,212    
Estimated gross benefit payments, 2020 11,428    
Estimated gross benefit payments, 2021 12,692    
Estimated gross benefit payments, 2022-2026 78,216    
Plan Assets [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Plan participants' contributions 2,774 $ 2,754  
Benefits paid (9,262) (8,430)  
Prescription drug rebates 865 890  
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Plan assets available for benefits at beginning of year 155,611 156,015  
Actual returns on plan assets 13,557 1,635  
Employer's contributions 0 2,747  
Plan participants' contributions 2,774 2,754  
Benefits paid (9,262) (8,430)  
Prescription drug rebates 865 890  
Plan assets available for benefits at end of year 163,545 155,611 $ 156,015
Fidelity Spartan US Equity Index Fund [Member] | Fair Value, Inputs, Level 1 [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Plan assets available for benefits at beginning of year 70,973    
Plan assets available for benefits at end of year 70,950 70,973  
Vanguard 500 Index Fund [Member] | Fair Value, Inputs, Level 1 [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Plan assets available for benefits at beginning of year 78,254    
Plan assets available for benefits at end of year 87,587 78,254  
Vanguard Total International Stock Member | Fair Value, Inputs, Level 1 [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Plan assets available for benefits at beginning of year 22,976    
Plan assets available for benefits at end of year 24,056 22,976  
Total Registered Investment Companies [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Trust liabilities (19,048) (16,592)  
Total Registered Investment Companies [Member] | Fair Value, Inputs, Level 1 [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Plan assets available for benefits at beginning of year 172,203    
Plan assets available for benefits at end of year $ 182,593 $ 172,203