Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

SHORT-TERM AND LONG-TERM DEBT - Short-Term Borrowing Limits (Details) - NSTAR Electric Company [Member] - USD ($)	Sep. 30, 2016	Aug. 08, 2016
Line of Credit Facility [Line Items]
Short-term borrowing limit approved by regulatory agency		$ 655,000,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 450,000,000