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FAIR VALUE (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair value:    
Regulatory liabilities $ 128,439 $ 137,369
Principal Amount    
Fair value:    
Marketable securities 594 818
Long-term debt 3,170,688 3,227,663
Estimated Fair Value    
Fair value:    
Cash and cash equivalents 92,200 107,200
Estimated Fair Value | Level 2    
Fair value:    
Long-term debt 3,682,338 3,421,753
Estimated Fair Value | Recurring | Level 1    
Fair value:    
Marketable securities $ 582 $ 712