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REGULATORY MATTERS (Regulatory Assets) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 1 year  
Regulatory Assets $ 400,000 $ 388,000
Regulatory Assets, Current 106,725 93,960
Regulatory Assets, Noncurrent 293,078 293,551
Pension and Other Postretirement Benefits    
Regulatory Assets [Line Items]    
Regulatory Assets 126,000 126,000
Early Generation Retirement Costs    
Regulatory Assets [Line Items]    
Regulatory Assets 72,000 84,000
Income Taxes Recoverable through Future Rates    
Regulatory Assets [Line Items]    
Regulatory Assets $ 47,000 40,000
Lost Fixed Cost Recovery    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 2 years  
Regulatory Assets $ 35,000 29,000
Final Mine Reclamation and Retiree Health Care Costs    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 20 years  
Regulatory Assets $ 29,000 31,000
Derivatives    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 11 years  
Regulatory Assets $ 27,000 18,000
Property Tax Deferrals    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 1 year  
Regulatory Assets $ 23,000 24,000
Springerville Unit 1 Leasehold Improvements    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 5 years  
Regulatory Assets $ 11,000 14,000
Other Regulatory Assets    
Regulatory Assets [Line Items]    
Regulatory Assets $ 30,000 $ 22,000