REGULATORY ACCOUNTING (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Regulatory Asset [Line Items]
Regulatory Assets Current	$ 241,902,000	$ 255,144,000
Regulatory Assets Long Term	3,214,208,000	3,267,710,000
Regulatory Assets, Total	3,456,100,000	3,522,800,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Description of Regulatory Costs Not yet Approved	Additionally, the Regulated companies had $32.6 million ($4.2 million for CL&P, $23.5 million for PSNH, and $1.6 million for WMECO) and $32.4 million ($5 million for CL&P, $22.4 million for PSNH, and $1.6 million for WMECO) of regulatory costs as of March 31, 2012 and December 31, 2011, respectively, which were included in Other Long-Term Assets on the accompanying unaudited condensed consolidated balance sheets. These amounts represent incurred costs that have not yet been approved for recovery by the applicable regulatory agency. Management believes these costs are probable of recovery in future cost-of-service regulated rates.
Business Aquisition, Description of Settlement Agreements	Regulatory Approvals: On February 15, 2012, NU and NSTAR reached comprehensive settlement agreements with the Massachusetts Attorney General and the DOER. The settlement agreement reached with the Attorney General covered a variety of rate-making and rate design issues, including a base distribution rate freeze at least through 2015 for NSTAR Electric, NSTAR Gas and WMECO and $15 million, $3 million and $3 million in the form of rate credits to their respective customers. The settlement agreement reached with the DOER covered the same rate-making and rate design issues as the Attorney General's settlement agreement, as well as a variety of matters impacting the advancement of Massachusetts clean energy policy established by the Green Communities Act and Global Warming Solutions Act. On April 4, 2012, the DPU issued a decision approving the settlement agreements and the merger of NU and NSTAR. On March 13, 2012, NU and NSTAR reached a comprehensive settlement agreement with both the Connecticut Attorney General and the Connecticut Office of Consumer Counsel. The settlement agreement covered a variety of matters, including a $25 million rate credit to CL&P customers, a CL&P base distribution rate freeze until December 1, 2014, and the establishment of a $15 million fund for energy efficiency and other initiatives to be disbursed at the direction of the DEEP. In the agreement, CL&P agreed to forego recovery of $40 million of the deferred storm costs associated with restoration activities following Tropical Storm Irene and the October 2011 snowstorm. Subject to the PURA review, the remaining storm costs are to be recovered during the six-year period beginning December 1, 2014. On April 2, 2012, the PURA issued a decision approving the settlement agreement and the merger of NU and NSTAR. The financial impacts of the settlement agreements will be recognized by NU, CL&P, NSTAR Electric, NSTAR Gas, and WMECO in the second quarter of 2012 in connection with the completion of the merger. As part of the settlement agreement NU and NSTAR reached with both the Connecticut Attorney General and the Connecticut Office of Consumer Counsel on March 13, 2012, in the second quarter of 2012, CL&P will record a reserve of $40 million associated with the deferred storm costs related to Tropical Storm Irene and the October 2011 snowstorm. CL&P will file with PURA for recovery of the total deferred storm costs. The total approved costs, which will reflect the $40 million reserve, will be collected over a six year period beginning December 1, 2014. Management believes CL&P's, PSNH's and WMECO's response to the 2011 storms was prudent and therefore believes it is probable that they will be allowed to recover these remaining deferred storm costs. See Note 2, “Merger of NU and NSTAR,” to the unaudited condensed consolidated financial statements for further information.
Regulatory Liabilities [Line Items]
Regulatory Liability Current	149,755,000	167,844,000
Regulatory Liabilities Long-Term	248,314,000	266,145,000
Regulatory Liabilities, Total	398,100,000	433,900,000
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	164,300,000	172,200,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	112,900,000	139,100,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	66,700,000	67,000,000
Pension Liability Yankee Gas Acquisition [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	9,400,000	10,000,000
Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	15,400,000	15,400,000
Wholesale Transmission Overcollections [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	9,000,000	9,600,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	20,400,000	20,600,000
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	172,277,000	170,197,000
Regulatory Assets Long Term	2,070,122,000	2,103,830,000
Regulatory Assets, Total	2,242,400,000	2,274,000,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	82,423,000	108,291,000
Regulatory Liabilities Long-Term	133,844,000	139,307,000
Regulatory Liabilities, Total	216,200,000	247,600,000
The Connecticut Light And Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	57,100,000	63,800,000
The Connecticut Light And Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	69,700,000	94,400,000
The Connecticut Light And Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	57,400,000	57,700,000
The Connecticut Light And Power Company [Member] | Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	15,400,000	15,400,000
The Connecticut Light And Power Company [Member] | Wholesale Transmission Overcollections [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	5,100,000	4,500,000
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	11,500,000	11,800,000
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	33,362,000	34,178,000
Regulatory Assets Long Term	375,122,000	393,941,000
Regulatory Assets, Total	408,500,000	428,100,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	23,108,000	24,500,000
Regulatory Liabilities Long-Term	54,212,000	54,415,000
Regulatory Liabilities, Total	77,300,000	78,900,000
Public Service Company Of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	52,800,000	53,200,000
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	14,000,000	17,300,000
Public Service Company Of New Hampshire [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	0	0
Public Service Company Of New Hampshire [Member] | Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	0	0
Public Service Company Of New Hampshire [Member] | Wholesale Transmission Overcollections [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,400,000	2,600,000
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	6,100,000	5,800,000
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	33,525,000	35,520,000
Regulatory Assets Long Term	242,181,000	233,247,000
Regulatory Assets, Total	275,700,000	268,700,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	30,389,000	33,056,000
Regulatory Liabilities Long-Term	15,731,000	16,597,000
Regulatory Liabilities, Total	46,100,000	49,700,000
Western Massachusetts Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	6,200,000	7,200,000
Western Massachusetts Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	17,700,000	21,300,000
Western Massachusetts Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	9,300,000	9,300,000
Western Massachusetts Electric Company [Member] | Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	0	0
Western Massachusetts Electric Company [Member] | Wholesale Transmission Overcollections [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	10,800,000	9,500,000
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,100,000	2,400,000
Deferred Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,322,200,000	1,360,500,000
Deferred Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	556,500,000	572,800,000
Deferred Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	194,100,000	200,000,000
Deferred Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	115,500,000	118,900,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	912,200,000	939,600,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	899,900,000	932,000,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Regulatory Assets Offsetting Derivative Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	12,300,000	7,300,000
Securitized Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	83,500,000	101,800,000
Securitized Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Securitized Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	62,500,000	76,400,000
Securitized Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	21,000,000	25,400,000
Deferred Income Tax Charges Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	446,100,000	425,400,000
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	348,000,000	339,600,000
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	37,700,000	38,000,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	29,300,000	17,800,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	95,400,000	100,900,000
Unrecovered Contractual Obligations [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	76,700,000	80,900,000
Unrecovered Contractual Obligations [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Unrecovered Contractual Obligations [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	18,700,000	20,000,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	43,700,000	45,900,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	4,700,000	5,500,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	14,500,000	11,900,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	22,700,000	22,100,000
Storm Cost Deferrals [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	365,500,000	356,000,000
Storm Cost Deferrals [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	280,900,000	268,300,000
Storm Cost Deferrals [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	41,700,000	44,000,000
Storm Cost Deferrals [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	42,900,000	43,700,000
Asset Retirement Obligation Costs Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	48,400,000	47,500,000
Asset Retirement Obligation Costs Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	28,500,000	27,900,000
Asset Retirement Obligation Costs Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	13,700,000	13,500,000
Asset Retirement Obligation Costs Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	3,300,000	3,200,000
Loss On Reacquired Debt Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	24,200,000	24,500,000
Loss On Reacquired Debt Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	13,900,000	13,900,000
Loss On Reacquired Debt Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	8,800,000	9,000,000
Loss On Reacquired Debt Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	300,000	300,000
Deferred Environmental Remediation Costs Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	37,600,000	38,500,000
Deferred Environmental Remediation Costs Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Deferred Environmental Remediation Costs Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	9,800,000	9,700,000
Deferred Environmental Remediation Costs Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	77,300,000	82,200,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	33,300,000	33,100,000
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	25,700,000	25,600,000
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 9,700,000	$ 10,000,000