INCOME TAXES - Net Deferred Tax Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets
Safe harbor lease receivables	$ 14,552	$ 17,067
Net operating loss carryforwards	116,797	100,565
Alternative minimum tax credit carryforwards	308	615
Deferred revenues and membership withdrawal	28,185	29,650
Operating lease liabilities	131,817
Other	26,587	22,483
Deferred tax assets, gross	318,246	170,380
Less valuation allowance	(30,468)
Deferred tax assets, net	287,778	170,380
Deferred tax liabilities
Basis differences- property, plant and equipment	129,427	115,887
Capital credits from other associations	32,789	32,689
Deferred debt prepayment transaction costs	33,542	35,595
Operating lease right-of-use assets	136,930
Other	14,027	4,307
Deferred tax liabilities, total	346,715	188,478
Net deferred tax liability	(58,937)	(18,098)
Deferred tax treatment of regulatory assets
Increase in net deferred tax liability not recognized as expense	40,800
Regulatory assets	497,279	437,377
Deferred income tax expense
Deferred tax treatment of regulatory assets
Regulatory assets	$ 58,937	$ 18,098