DERIVATIVES (Volumes of Outstanding Derivative Contracts, in Megawatt Hours Unless Otherwise Specified) (Details)	Sep. 30, 2018 MWh MMBTU	Dec. 31, 2017 MWh MMBTU
Forwards, Futures and Swaps \| Natural gas
Derivative [Line Items]
Contract Volume \| MMBTU	250,021,802	228,768,745
Forwards, Futures and Swaps \| Electric
Derivative [Line Items]
Contract Volume \| MWh	3,939,691	2,872,013
Options \| Natural gas
Derivative [Line Items]
Contract Volume \| MMBTU	29,534,224	60,736,806
Congestion Revenue Rights \| Electric
Derivative [Line Items]
Contract Volume \| MWh	316,451,690	312,272,177

The number of derivative instruments of a particular group held by the entity.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Name:	us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant