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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Name:	us-gaap_UnusualOrInfrequentItemLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

UTILITY PLANT AND JOINTLY-OWNED FACILITIES (Springerville Acquisition) (Details) - Springerville Unit One	Dec. 31, 2017	Dec. 31, 2016	Sep. 16, 2016
Unusual or Infrequent Item, or Both [Line Items]
Percentage of ownership in generating units (in percentage)	32.20%	100.00%
Completion of Purchase of Equity Interest
Unusual or Infrequent Item, or Both [Line Items]
Percentage of ownership in generating units (in percentage)			100.00%