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DERIVATIVE INSTRUMENTS (Schedule of Notional Amounts of Outstanding Derivatives) (Details)
gal in Millions, MMcf in Millions
Jun. 30, 2019
gal
MMcf
Dec. 31, 2018
gal
MMcf
Natural Gas [Member]    
Derivative [Line Items]    
Investment Contract Volume | MMcf 9.0 1.8
Diesel [Member]    
Derivative [Line Items]    
Investment Contract Volume | gal 5.0 11.0