INCOME TAXES - Effective Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation between the statutory federal income tax rate and the effective tax rate
Federal income tax expense at statutory rate	21.00%	21.00%	35.00%
State income tax expense, net of federal benefit	2.80%	2.80%	2.63%
Patronage exclusion	(23.80%)	(23.80%)	(37.63%)
Asset retirement obligations	(11.33%)	3.57%	(0.16%)
Deferred revenues and membership withdrawal	3.23%	(28.78%)	5.11%
Operating liabilities, net of right-of-use assets	11.29%
Valuation allowance	67.24%
Various book tax differences	(2.43%)	24.42%	(2.82%)
Regulatory treatment of deferred taxes	(68.68%)	(0.46%)	(3.91%)
Effective tax rate	(0.68%)	(1.25%)	(1.78%)