COMMITMENTS AND CONTINGENCIES (CONTINGENCIES) (Detail) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	May 30, 2017	Jan. 31, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies [Line Items]
Payments to Acquire Other Productive Assets				$ 0	$ 85,000	$ 45,753
Loss Contingency, Accrual, Reversal	$ 5,000
Springerville Unit 1 Third Party Owner Allegation | Settled Litigation
Commitments And Contingencies [Line Items]
Payments to Acquire Other Productive Assets			$ 85,000
Proceeds from Legal Settlements			$ 12,500
FERC Refund Orders
Commitments And Contingencies [Line Items]
Litigation settlement, amount awarded from other party		$ 8,000
FERC Refund Orders | Wholesale Revenue
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]
Share of Reclamation Costs Anticipated				61,000
Navajo, San Juan, Four Corners | Other Liabilities
Commitments And Contingencies [Line Items]
Environmental Exit Costs, Costs Accrued to Date				$ 34,000	$ 26,000