COMMITMENTS AND CONTINGENCIES (CONTINGENCIES) (Detail) $ in Thousands	1 Months Ended			12 Months Ended
	Jan. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Feb. 29, 2016 USD ($)	Dec. 31, 2016 USD ($) mine state	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Commitments And Contingencies [Line Items]
Payments to Acquire Other Productive Assets				$ 85,000	$ 45,753	$ 19,608
Springerville Unit 1 Third Party Owner Allegation [Member] | Settled Litigation [Member]
Commitments And Contingencies [Line Items]
Jointly Owned Utility Plant Proportionate Ownership Share, Purchased			50.50%
Purchase Obligation			$ 85,000
Litigation Settlement Amount			$ 12,500
Payments to Acquire Other Productive Assets		$ 85,000
Percentage Of Ownership In Generating Units		100.00%
Proceeds from Legal Settlements		$ 12,500
FERC Refund Orders [Member]
Commitments And Contingencies [Line Items]
Loss Contingency Accrual, Provision				22,000
Loss Contingency Accrual, Payments				17,000
FERC Refund Orders [Member] | Wholesale Revenue [Member]
Commitments And Contingencies [Line Items]
Loss Contingency Accrual, Provision				22,000
FERC Refund Orders [Member] | Other Current Liabilities [Member]
Commitments And Contingencies [Line Items]
Loss Contingency Accrual				$ 5,000
FERC Refund Orders [Member] | Subsequent Event [Member]
Commitments And Contingencies [Line Items]
Litigation Settlement Amount	$ 8,000
San Juan Generating Station and WildEarth Guardians Allegation [Member]
Commitments And Contingencies [Line Items]
Loss Contingency Number of Mines | mine				7
Loss Contingency Number of States | state				4
Navajo [Member]
Commitments And Contingencies [Line Items]
Percentage of ownership in generating station				7.50%
Loss Contingency Accrual, Period Increase (Decrease)				$ 5,000
Navajo [Member] | Other Noncurrent Liabilities [Member]
Commitments And Contingencies [Line Items]
Loss Contingency Accrual				3,000
Navajo, San Juan, Four Corners [Member]
Commitments And Contingencies [Line Items]
Share of Reclamation Costs Anticipated				61,000
Navajo, San Juan, Four Corners [Member] | Other Liabilities [Member]
Commitments And Contingencies [Line Items]
Environmental Exit Costs, Costs Accrued to Date				$ 26,000	$ 25,000
Gila [Member]
Commitments And Contingencies [Line Items]
Percentage of ownership in generating station				75.00%
Gila [Member] | UNS Electric [Member]
Commitments And Contingencies [Line Items]
Percentage of ownership in generating station				25.00%