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FAIR VALUE (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Fair value:    
Regulatory liabilities $ 122,169 $ 137,369
Principal Amount    
Fair value:    
Marketable securities 715 818
Long-term debt 3,166,472 3,227,663
Estimated Fair Value    
Fair value:    
Cash and cash equivalents 79,000 107,200
Estimated Fair Value | Level 2    
Fair value:    
Long-term debt 3,608,341 3,421,753
Estimated Fair Value | Recurring | Level 1    
Fair value:    
Marketable securities $ 654 $ 712