REGULATORY MATTERS (Regulatory Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 465,000	$ 400,000
Regulatory Assets	138,412	106,725
Regulatory Assets	326,860	293,078
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 135,000	126,000
Derivatives
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	10 years
Total Regulatory Assets	$ 72,000	27,000
Early Generation Retirement Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 68,000	72,000
Lost Fixed Cost Recovery
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	2 years
Total Regulatory Assets	$ 46,000	35,000
Income Taxes Recoverable through Future Rates
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 38,000	47,000
Under Recovered Purchased Energy Costs
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 36,000	0
Property Tax Deferrals
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 24,000	23,000
Remaining recovery period (in years)	6 months
Final Mine Reclamation and Retiree Healthcare Costs
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	19 years
Total Regulatory Assets	$ 19,000	29,000
Springerville Unit 1 Leasehold Improvements
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	4 years
Total Regulatory Assets	$ 9,000	11,000
Useful life (in years)	10 years
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 18,000	$ 30,000