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EMPLOYEE BENEFITS - Postretirement Benefits (Details) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Aug. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Profit Sharing Automatic Contribution By Employer, Percentage |  | 3.00% |  |
Decrease in postretirement benefit obligation | $ 75,700 |  |  |
Postretirement benefit plan re-measurement, tax | 29,200 |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |
Plan amendment | $ (75,700) |  |  |
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] |  |  |  |
HRA credit inflation index for grandfathered retirees |  | 0.0250 |  |
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract] |  |  |  |
Pension and Other Postretirement Benefits Cost (Reversal of Cost) |  | $ 120,000 | $ 84,000 | $ 121,000
Defined Contribution Plan, Cost |  | 18,000 | 12,000 | 11,000
Postretirement Benefits [Member] |  |  |  |
Decrease in postretirement benefit obligation |  | 34,182 | 0 |
Postretirement Benefits |  |  |  |
Service cost |  | 7,423 | 8,238 | 10,128
Interest cost |  | 8,103 | 9,855 | 9,649
Expected return on assets |  | (11,826) | (10,113) | (10,375)
Amortization of prior service credits |  | (7,570) | (6,688) | (6,801)
Amortization of unrecognized losses |  | (2,437) | 129 | 1,512
Net periodic benefit costs |  | (6,307) | 1,421 | 4,113
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |
Benefit obligation at beginning of year |  | 265,028 | 239,348 |
Service cost |  | 7,423 | 8,238 | 10,128
Interest cost |  | 8,103 | 9,855 | 9,649
Plan participants' contributions |  | 3,346 | 2,943 |
Actuarial (gains) losses |  | (34,236) | 13,218 |
Plan amendment |  | (34,182) | 0 |
Benefits paid |  | (8,631) | (9,439) |
Prescription drug rebates |  | 1,499 | 865 |
Benefit obligation at end of year |  | 208,350 | 265,028 | $ 239,348
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |
Prescription drug rebates |  | 1,499 | 865 |
Noncurrent postretirement benefit obligation |  | 18,858 | 101,483 |
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract] |  |  |  |
Prior service credit |  | 80,426 | 53,814 |
Unrecognized gains (losses) |  | 36,794 | (12,656) |
Deferred tax (liability) |  | (43,793) | (15,861) |
Net accumulated gains |  | 73,427 | $ 25,297 |
Increase in unrecognized Losses in AOCE |  | 49,000 |  |
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 |  | (9,696) |  |
Amortization of unrecognized gains |  | (2,787) |  |
Estimated amount to be amortized from AOCE into net periodic postretirement benefit costs |  | $ (12,483) |  |
Net unrecognized gains (losses), amortization period |  | 13 years 2 months 12 days |  |
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] |  |  |  |
Discount rate |  | 4.00% | 4.20% | 3.89%
Long-term rate of return on plan assets, net of tax |  | 7.13% | 6.65% | 6.65%
Initial healthcare cost trend rate - pre age 65 |  | 0.0681 | 0.0700 | 0.0725
Initial healthcare cost trend rate, post age 65 |  | 0.0936 | 0.0700 | 0.0725
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 |  | 3.44% | 4.00% | 4.20%
Expected long-term rate of return on plan assets, net of tax |  | 7.13% | 7.13% | 6.65%
Initial healthcare cost trend rate - pre age 65 |  | 6.56% | 6.81% | 7.00%
Initial healthcare cost trend rate - post age 65 |  |  | 9.36% | 7.00%
Catastrophic drug benefit |  | 0.1250 |  |
Ultimate healthcare cost trend rate |  | 4.50% | 4.50% | 4.50%
Year ultimate healthcare cost trend rate reached |  | 2026 | 2026 | 2026
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract] |  |  |  |
Effect of one percentage point increase on service and interest cost |  | $ 1,269 |  |
Effect of one percentage point decrease on service and interest cost |  | (1,053) |  |
Effect of one percentage point increase on postretirement benefit obligation |  | 5,516 |  |
Effect of one percentage point decrease on postretirement benefit obligation |  | (5,095) |  |
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract] |  |  |  |
Estimated gross benefit payments, 2018 |  | 7,365 |  |
Estimated gross benefit payments, 2019 |  | 8,956 |  |
Estimated gross benefit payments, 2020 |  | 10,029 |  |
Estimated gross benefit payments, 2021 |  | 11,025 |  |
Estimated gross benefit payments, 2022 |  | 12,077 |  |
Estimated gross benefit payments, 2023-2027 |  | 67,760 |  |
Postretirement Benefits [Member] | Plan Assets [Member] |  |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |
Prescription drug rebates |  | 1,499 | $ 865 |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |
Plan assets available for benefits at beginning of year |  | 163,545 | 155,611 |
Actual returns on plan assets |  | 29,477 | 13,557 |
Plan participants' contributions |  | 3,266 | 2,774 |
Benefits paid |  | (8,295) | (9,262) |
Prescription drug rebates |  | 1,499 | 865 |
Plan assets available for benefits at end of year |  | 189,492 | 163,545 | $ 155,611
Postretirement Benefits [Member] | 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 |  | 70,950 |  |
Plan assets available for benefits at end of year |  | 83,238 | 70,950 |
Postretirement Benefits [Member] | 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 |  | 87,587 |  |
Plan assets available for benefits at end of year |  | 103,706 | 87,587 |
Postretirement Benefits [Member] | 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 |  | 24,056 |  |
Plan assets available for benefits at end of year |  | 30,684 | 24,056 |
Postretirement Benefits [Member] | Total Registered Investment Companies [Member] |  |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |
Trust liabilities |  | (28,136) | (19,048) |
Postretirement Benefits [Member] | 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 |  | 182,593 |  |
Plan assets available for benefits at end of year |  | $ 217,628 | $ 182,593 |
Minimum [Member] |  |  |  |
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract] |  |  |  |
Profit Sharing Contribution Percentage |  | 8.00% |  |
Maximum [Member] |  |  |  |
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract] |  |  |  |
Profit Sharing Contribution Percentage |  | 18.00% |  |

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- Definition Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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