REVENUES - Future revenues from remaining performance obligations (Details)	6 Months Ended
Jun. 30, 2018
Revenue, practical expedient
Revenue, Remaining Performance Obligation, Optional Exemption, Performance Obligation	true
Minimum
Revenue performance
Current rate settlement term	1 year
Maximum
Revenue performance
Current rate settlement term	4 years

Indicates (true/false) whether practical expedient is elected not to disclose amount of transaction price allocated to remaining performance obligation and explanation of expected timing of revenue recognition, when contract has expected duration of one year or less or right to consideration corresponds directly to performance completed.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130558-203045

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130557-203045

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 270
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=84173487&loc=SL51747714-108306

Name:	us-gaap_RevenuePracticalExpedientRemainingPerformanceObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration