COMMITMENTS AND CONTINGENCIES (Contingencies) (Detail) $ in Thousands	1 Months Ended			12 Months Ended
	May 30, 2017 USD ($)	Jan. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) mine	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments And Contingencies [Line Items]
Purchase of third parties undivided interest				$ 0	$ 0	$ 85,000
Springerville Unit 1 Third Party Owner Allegation | Settled Litigation
Commitments And Contingencies [Line Items]
Purchase of third parties undivided interest			$ 85,000
Proceeds from third party			$ 12,500
FERC Refund Orders
Commitments And Contingencies [Line Items]
Litigation settlement, amount awarded from other party		$ 8,000
Reversal of loss contingency accrual	$ 5,000
FERC Refund Orders | Wholesale
Commitments And Contingencies [Line Items]
Loss contingency accrual, provision						22,000
FERC Refund Orders | Other Current Liabilities
Commitments And Contingencies [Line Items]
Loss contingency accrual						$ 5,000
Navajo, San Juan, Four Corners
Commitments And Contingencies [Line Items]
Number of mines expected to be reclaimed | mine				3
Reclamation costs				$ 66,000
Navajo, San Juan, Four Corners | Other Liabilities
Commitments And Contingencies [Line Items]
Reclamation costs accrued				$ 36,000	$ 34,000