REGULATORY ACCOUNTING - Goodwill-related (Details) - NSTAR Electric and NSTAR Gas [Member] - Goodwill-related [Member]	12 Months Ended
Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	40 years
Regulatory Asset, Amortization Period Remaining	23 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