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REGULATORY MATTERS (Regulatory Assets) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 1 year  
Regulatory Assets $ 281,000 $ 264,000
Regulatory Assets, Current 56,340 51,841
Regulatory Assets, Noncurrent 225,453 212,312
Pension and Other Postretirement Benefits (Note 8)    
Regulatory Assets [Line Items]    
Regulatory Assets 128,000 120,000
Income Taxes Recoverable through Future Rates (1)    
Regulatory Assets [Line Items]    
Regulatory Assets $ 29,000 26,000
Final Mine Reclamation and Retiree Health Care Costs (2)    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 21 years  
Regulatory Assets $ 27,000 28,000
Property Tax Deferrals (3)    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 6 months  
Regulatory Assets $ 23,000 21,000
Lost Fixed Cost Recovery    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 1 year  
Regulatory Assets $ 23,000 16,000
Springerville Unit 1 Leasehold Improvements (4)    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 7 years  
Regulatory Assets $ 17,000 21,000
Amortization period authorized to recover leasehold improvement costs at Springerville Unit 1 10 years  
Sundt Coal Handling Facilities (5)    
Regulatory Assets [Line Items]    
Regulatory Assets $ 16,000 0
Derivatives (Note 11)    
Regulatory Assets [Line Items]    
Remaining Recovery Period (years) 3 years  
Regulatory Assets $ 2,000 12,000
Other Regulatory Assets    
Regulatory Assets [Line Items]    
Regulatory Assets $ 16,000 $ 20,000