INCOME TAXES - Net Deferred Tax Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets
Safe harbor lease receivables	$ 35,945	$ 38,218
Net operating loss carryforwards	152,936	131,935
Alternative minimum tax credit carryforwards	3,834	3,834
Deferred Tax Assets, Deferred Income	16,933	16,933
Colowyo Coal- coal contract intangible liability	2,341	3,547
Other	35,332	46,856
Deferred tax assets, total	247,321	241,323
Deferred tax liabilities
Basis differences- property, plant and equipment	160,494	152,497
Capital credits from other associations	38,663	36,790
Deferred debt prepayment transaction costs	66,020	69,266
Other	10,773
Deferred tax liabilities, total	275,950	258,553
Net deferred tax liability	(28,629)	(17,230)
Deferred tax treatment of regulatory assets
Regulatory assets	415,081	426,043
Deferred income tax expense
Deferred tax treatment of regulatory assets
Increase in net deferred tax liability not recognized as expense	11,400
Regulatory assets	$ 28,629	$ 17,230