REGULATORY MATTERS (Regulatory Assets and liabilities- Footnotes) (Details)	12 Months Ended
Dec. 31, 2018
Regulatory Assets [Line Items]
Remaining recovery period (years)	1 year
Property Tax Pay-as -you-go or Cash Basis
Regulatory Assets [Line Items]
Remaining recovery period (years)	6 months
Springerville Unit 1 Leasehold Improvements
Regulatory Assets [Line Items]
Remaining recovery period (years)	5 years
Amortization period authorized to recover leasehold improvement costs at Springerville Unit 1	10 years

Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 55
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82849221&loc=d3e44360-110383

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

Name:	us-gaap_RemainingRecoveryPeriodOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration