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SUBSEQUENT EVENTS (Details) - USD ($)
$ in Thousands
1 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Subsequent Event [Line Items]      
Principal amount   $ 106,931 $ 103,647
First Mortgage Bonds [Member]      
Subsequent Event [Line Items]      
Principal amount   $ 99,600 $ 95,600
Subsequent Event [Member] | First Mortgage Bonds [Member]      
Subsequent Event [Line Items]      
Principal amount $ 25,000    
Subsequent Event [Member] | First Mortgage Bonds [Member] | Series U [Member]      
Subsequent Event [Line Items]      
Principal amount $ 25,000    
Maturity date Jan. 31, 2038    
Interest rate, stated percentage 4.71%