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REGULATORY MATTERS (Regulatory Assets) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 1 year  
Regulatory Assets $ 388,000 $ 281,000
Regulatory Assets, Current 93,960 56,340
Regulatory Assets, Noncurrent 293,551 225,453
Pension and Other Postretirement Benefits (Note 8)    
Regulatory Assets [Line Items]    
Regulatory Assets 126,000 128,000
Early Generation Retirement Costs    
Regulatory Assets [Line Items]    
Regulatory Assets 84,000 0
Income Taxes Recoverable through Future Rates    
Regulatory Assets [Line Items]    
Regulatory Assets $ 40,000 29,000
Final Mine Reclamation and Retiree Health Care Costs    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 20 years  
Regulatory Assets $ 31,000 27,000
Lost Fixed Cost Recovery    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 1 year  
Regulatory Assets $ 29,000 23,000
Property Tax Deferrals    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 1 year  
Regulatory Assets $ 24,000 23,000
Springerville Unit 1 Leasehold Improvements    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 6 years  
Regulatory Assets $ 14,000 17,000
Sundt Coal Handling Facilities    
Regulatory Assets [Line Items]    
Regulatory Assets 0 14,000
Other Regulatory Assets    
Regulatory Assets [Line Items]    
Regulatory Assets $ 40,000 $ 20,000