REGULATORY MATTERS (Regulatory Assets) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 404,000	$ 388,000
Less Current Portion	110,329	93,960
Total Non-Current Regulatory Assets	293,627	293,551
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets	121,000	126,000
Early Generation Retirement Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	74,000	84,000
Income Taxes Recoverable through Futures Rates [Member] [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 43,000	40,000
Derivatives
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	3 years
Total Regulatory Assets	$ 42,000	18,000
Final Mine Reclamation and Retiree Healthcare Costs
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	19 years
Total Regulatory Assets	$ 33,000	31,000
Lost Fixed Cost Recovery
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	2 years
Total Regulatory Assets	$ 32,000	29,000
Property Tax Deferrals
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 25,000	24,000
Springerville Unit 1 Leasehold Improvements
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	5 years
Total Regulatory Assets	$ 12,000	14,000
Useful life (in years)	10 years
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 22,000	$ 22,000