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

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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