SHORT TERM DEBT (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Line of Credit Facility [Line Items]
Letters Of Credit Outstanding Under Line of Credit	$ 17.9	$ 32.1
NU Money Pool [Abstract]
Short-term Debt, Terms	CL&P, PSNH, WMECO and Yankee Gas Credit Agreement: On September 24, 2010, CL&P, PSNH, WMECO and Yankee Gas jointly entered into a three-year unsecured revolving credit facility in the amount of $400 million, which terminates on September 24, 2013. CL&P and PSNH may borrow up to $300 million each under this facility, with WMECO and Yankee Gas able to borrow up to $200 million each, subject to the $400 million maximum aggregate borrowing limit. This total commitment may be increased to $500 million at the request of the borrowers, subject to lender approval. Under this facility, each company can borrow either on a short-term or a long-term basis subject to regulatory approval. NU Parent Credit Agreement:  On September 24, 2010, NU parent entered into a three-year unsecured revolving credit facility in the amount of $500 million, which terminates on September 24, 2013. Subject to the amount of advances outstanding, LOCs can be issued under this facility for periods up to 364 days on the account of NU parent or any of its subsidiaries up to the total amount of the facility. This total commitment may be increased to $600 million at the request of NU parent, subject to lender approval. Under this facility, NU parent can borrow either on a short-term or a long-term basis.
Short Term Debt Regulatory Limits	:  The amount of short-term borrowings that may be incurred by CL&P and WMECO is subject to periodic approval by the FERC. As a result of the NHPUC having jurisdiction over PSNH's short-term debt, PSNH is not currently required to obtain FERC approval for its short-term borrowings. On November 30, 2011, the FERC granted authorization to allow CL&P and WMECO to incur total short-term borrowings up to a maximum of $450 million and $300 million, respectively, effective January 1, 2012 through December 31, 2013. PSNH is authorized by regulation of the NHPUC to incur short-term borrowings up to 10 percent of net fixed plant. In an order dated December 17, 2010, the NHPUC increased the amount of short-term borrowings authorized for PSNH to a maximum of 10 percent of net fixed plant plus an additional $60 million until further ordered by the NHPUC. As of December 31, 2011, PSNH's short-term debt authorization under the 10 percent of net fixed plant test plus $60 million totaled approximately $270 million. CL&P's certificate of incorporation contains preferred stock provisions restricting the amount of unsecured debt that CL&P may incur, including limiting unsecured indebtedness with a maturity of less than 10 years to 10 percent of total capitalization. In November 2003, CL&P obtained from its preferred stockholders a waiver of such 10 percent limit for a ten-year period expiring in March 2014, provided that all unsecured indebtedness does not exceed 20 percent of total capitalization. As of December 31, 2011, CL&P had $826.3 million of unsecured debt capacity available under this authorization.
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding	31.0
Letters Of Credit Outstanding Under Line of Credit	4.0
NU Money Pool [Abstract]
Borrowings From Money Pool	58.5	6.2
Weighted Average Interest Rate on Money Pool Borrowings	0.08%	0.19%
Short-term Debt, Weighted Average Interest Rate	4.03%
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding		30.0
Letters Of Credit Outstanding Under Line of Credit	5.4	30.1
NU Money Pool [Abstract]
Borrowings From Money Pool		47.9
Contributions to NU Money Pool	(55.9)
Weighted Average Interest Rate on Money Pool Borrowings	0.10%	0.18%
Short-term Debt, Weighted Average Interest Rate		2.05%
Western Massachusetts Electric Company [Member]
NU Money Pool [Abstract]
Borrowings From Money Pool		20.4
Contributions to NU Money Pool	(11.0)
Weighted Average Interest Rate on Money Pool Borrowings	0.10%	0.14%
Nu Parent [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding	256.0	237.0
NU Money Pool [Abstract]
Short-term Debt, Weighted Average Interest Rate	2.20%	2.85%
Yankee Gas Services Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding	$ 30.0
NU Money Pool [Abstract]
Short-term Debt, Weighted Average Interest Rate	2.07%