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VARIABLE INTEREST ENTITIES - Consolidated Entities (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Consolidated Variable Interest Entities      
Net electric plant $ 3,613,846 $ 3,607,484  
Noncontrolling interest 111,717 110,169  
Long-term debt 3,063,351 3,109,301  
Accrued interest 29,716 32,070  
Interest expense $ 151,470 153,704 $ 147,608
Springerville Partnership      
Consolidated Variable Interest Entities      
Equity interest (as a percent) 51.00%    
General partner interest (as a percent) 1.00%    
Springerville Partnership | Springerville Unit 3 Lease      
Consolidated Variable Interest Entities      
Percentage of financial and other support provided 100.00%    
Springerville Partnership | Reportable Legal Entities      
Consolidated Variable Interest Entities      
Net electric plant $ 776,411 794,549  
Noncontrolling interest 111,717 110,169  
Long-term debt 380,867 416,057  
Accrued interest 11,050 12,056  
Depreciation and amortization 18,138 18,138 19,592
Interest expense $ 25,320 $ 27,511 $ 28,382
Springerville Partnership | Springerville Unit 3 Holding LLC      
Consolidated Variable Interest Entities      
Ownership interest held by parent (as a percent) 100.00%    
Other Limited Partners | Springerville Partnership      
Consolidated Variable Interest Entities      
Ownership interest held by noncontrolling interest (as a percent) 49.00%