LONG-TERM DEBT (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Long-Term Debt	$ 7,011,561	$ 7,200,156
The Connecticut Light And Power Company [Member]
Debt Instrument [Line Items]
Long-Term Debt	2,740,614	2,737,790
NSTAR Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,600,950	1,600,911
Public Service Company Of New Hampshire [Member]
Debt Instrument [Line Items]
Long-Term Debt	888,999	997,932
Western Massachusetts Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt	$ 550,057	$ 550,270

Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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