COMMITMENTS AND CONTINGENCIES - Guarantees and Indemnifications Narrative (Details) $ in Millions	Mar. 31, 2017 USD ($)
Access Northeast [Member]
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure (up to)	$ 206
Guarantee Of Financial Obligations Of Npt [Member]
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure (up to)	25
Guarantee Of Npt Letters Of Credit [Member]
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure (up to)	$ 14

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

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