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FAIR VALUE (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Fair value:    
Regulatory liabilities $ 237,187 $ 122,169
Principal Amount    
Fair value:    
Marketable securities 477 715
Long-term debt 3,314,353 3,166,472
Estimated Fair Value    
Fair value:    
Cash and cash equivalents 102,200 79,000
Estimated Fair Value | Level 2    
Fair value:    
Long-term debt 3,926,312 3,608,341
Estimated Fair Value | Recurring | Level 1    
Fair value:    
Marketable securities $ 429 $ 654