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FAIR VALUE (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Fair value:    
Regulatory liabilities $ 122,051 $ 122,169
Principal Amount    
Fair value:    
Marketable securities 528 715
Long-term debt 3,324,478 3,166,472
Estimated Fair Value    
Fair value:    
Cash and cash equivalents 114,800 79,000
Estimated Fair Value | Level 2    
Fair value:    
Long-term debt 4,140,708 3,608,341
Estimated Fair Value | Recurring | Level 1    
Fair value:    
Marketable securities $ 400 $ 654