REGULATORY ACCOUNTING - Goodwill-related (Details) - NSTAR Electric and NSTAR Gas - Goodwill-related	12 Months Ended
Dec. 31, 2017
Regulatory Assets [Line Items]
Amortization period	40 years
Amortization period remaining	22 years

Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	us-gaap_RegulatoryAssetAmortizationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration