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INCOME TAXES - Effective Tax Rate Reconciliation (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation between the statutory federal income tax rate and the effective tax rate      
Federal income tax expense at statutory rate 35.00% 35.00% 35.00%
State income tax expense, net of federal benefit 2.63% 2.63% 2.63%
Patronage exclusion (37.63%) (37.63%) (37.63%)
Asset retirement obligations (0.16%) 5.85% (0.58%)
Postretirement medical actuarial gains and losses 0.02% 1.04% (1.09%)
Various book tax differences 2.27% (7.66%) 2.84%
Regulatory treatment of deferred taxes (3.91%) (3.79%) (1.17%)
Effective tax rate (1.78%) (4.56%) 0.00%