FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Derivative Volumes) (Details)	Sep. 30, 2018 GWh BTU	Dec. 31, 2017 GWh BTU
Power Contracts GWh
Derivative Volume [Line Items]
Derivatives volumes \| GWh	2,499	2,589
Gas Contracts BBtu
Derivative Volume [Line Items]
Derivatives volumes \| BTU	146,612,000,000,000	137,952,000,000,000

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant