COMMITMENTS AND CONTINGENCIES (Contingencies) (Details) - Navajo, San Juan, Four Corners - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Commitments And Contingencies [Line Items]
Share of reclamation costs anticipated	$ 65
Other Liabilities
Commitments And Contingencies [Line Items]
Environmental exit costs, costs accrued to date	$ 35	$ 34

Carrying amount as of the balance sheet date of the liability for remediation, site restoration, postclosure, monitoring commitments, or other exit costs associated with the sale, disposal or abandonment of a particular property resulting from unanticipated contamination of the assets.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 5.Y.Q4)
-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

Name:	us-gaap_EnvironmentalExitCostsCostsAccruedToDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant