The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3291-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

Name:	us-gaap_RepaymentsOfLongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

LONG-TERM DEBT - Long-Term Debt Repayment (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Repayments of debt		$ 200,000	$ 216,700	$ 576,551
NSTAR Electric Company [Member]
Debt Instrument [Line Items]
Repayments of debt		$ 200,000	$ 4,700	$ 301,650
NSTAR Electric Company [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Repayments of debt	$ 200,000