INCOME TAXES - Net Deferred Tax Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets
Safe harbor lease receivables		$ 19,222	$ 33,355
Net operating loss carryforwards		104,102	142,286
Alternative minimum tax credit carryforwards		1,230	2,417
Deferred revenues and membership withdrawal		17,350	30,583
Colowyo Coal- coal contract intangible liability			1,228
Other		23,707	41,066
Deferred tax assets, total		165,611	250,935
Deferred tax liabilities
Basis differences- property, plant and equipment		112,285	162,928
Capital credits from other associations		28,787	44,268
Deferred debt prepayment transaction costs		37,649	62,773
Other		4,095	11,483
Deferred tax liabilities, total		182,816	281,452
Net deferred tax liability		(17,205)	$ (30,517)
Effect of Tax Cuts and Jobs Act of 2017
Estimated income tax benefit for remeasurement of deferred tax from change in tax rate pursuant to Tax Cuts and Jobs Act of 2017		$ 17,200
Federal income tax statutory rate		35.00%	35.00%	35.00%
Deferred tax treatment of regulatory assets
Decrease in net deferred tax liability not recognized as benefit		$ 13,300
Regulatory assets		454,523	$ 395,615
Forecast
Effect of Tax Cuts and Jobs Act of 2017
Federal income tax statutory rate	21.00%
Deferred income tax expense
Deferred tax treatment of regulatory assets
Regulatory assets		$ 17,205	$ 30,517