REGULATORY ACCOUNTING (Details) (USD $)	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Regulatory Asset Line Items
Regulatory Assets Current	$ 235,460,000	$ 238,699,000
Regulatory Assets Long Term	2,706,053,000	2,756,580,000
Regulatory Assets, Total	2,941,600,000	2,995,300,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount Of Regulatory Costs Not Yet Approved	11,900,000	37,500,000
Regulatory Liabilities Line Items
Regulatory Liability Current	174,562,000	99,403,000
Regulatory Liabilities Long-Term	276,222,000	339,655,000
Regulatory Liabilities, Total	450,800,000	439,100,000
The Connecticut Light and Power Company [Member] | Deferred Benefit Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	432,900,000	471,800,000
Public Service Company of New Hampshire [Member] | Deferred Benefit Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	141,400,000	152,600,000
Western Massachusetts Electric Company [Member] | Deferred Benefit Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	88,600,000	96,000,000
Deferred Benefit Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	1,006,600,000	1,094,200,000
The Connecticut Light and Power Company [Member] | Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	891,600,000	846,200,000
Public Service Company of New Hampshire [Member] | Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	3,300,000	12,800,000
Western Massachusetts Electric Company [Member] | Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	895,300,000	859,700,000
The Connecticut Light and Power Company [Member] | Securitized Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Public Service Company of New Hampshire [Member] | Securitized Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	90,100,000	129,800,000
Western Massachusetts Electric Company [Member] | Securitized Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	29,600,000	41,900,000
Securitized Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	119,700,000	171,700,000
The Connecticut Light and Power Company [Member] | Deferred Income Tax Charges Member
Regulatory Asset Line Items
Regulatory Assets, Total	333,500,000	328,900,000
Public Service Company of New Hampshire [Member] | Deferred Income Tax Charges Member
Regulatory Asset Line Items
Regulatory Assets, Total	37,600,000	31,400,000
Western Massachusetts Electric Company [Member] | Deferred Income Tax Charges Member
Regulatory Asset Line Items
Regulatory Assets, Total	16,900,000	16,800,000
Deferred Income Tax Charges Member
Regulatory Asset Line Items
Regulatory Assets, Total	417,100,000	401,500,000
The Connecticut Light and Power Company [Member] | Unrecovered Contractual Obligations [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	85,400,000	97,900,000
Public Service Company of New Hampshire [Member] | Unrecovered Contractual Obligations [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Western Massachusetts Electric Company [Member] | Unrecovered Contractual Obligations [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	21,500,000	25,300,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	106,900,000	123,200,000
The Connecticut Light and Power Company [Member] | Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	7,100,000	35,500,000
Public Service Company of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	2,300,000	14,700,000
Western Massachusetts Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	15,900,000	15,200,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	30,400,000	70,300,000
The Connecticut Light and Power Company [Member] | Storm Cost Deferrals [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	104,900,000	4,000,000
Public Service Company of New Hampshire [Member] | Storm Cost Deferrals [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	46,200,000	40,700,000
Western Massachusetts Electric Company [Member] | Storm Cost Deferrals [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	20,600,000	15,400,000
Storm Cost Deferrals [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	171,700,000	60,100,000
The Connecticut Light and Power Company [Member] | Asset Retirement Obligation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	26,600,000	24,900,000
Public Service Company of New Hampshire [Member] | Asset Retirement Obligation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	15,100,000	14,700,000
Western Massachusetts Electric Company [Member] | Asset Retirement Obligation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	3,200,000	3,000,000
Asset Retirement Obligation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	47,700,000	45,300,000
The Connecticut Light and Power Company [Member] | Loss On Reacquired Debt Member
Regulatory Asset Line Items
Regulatory Assets, Total	10,900,000	11,200,000
Public Service Company of New Hampshire [Member] | Loss On Reacquired Debt Member
Regulatory Asset Line Items
Regulatory Assets, Total	9,200,000	8,400,000
Western Massachusetts Electric Company [Member] | Loss On Reacquired Debt Member
Regulatory Asset Line Items
Regulatory Assets, Total	300,000	400,000
Loss On Reacquired Debt Member
Regulatory Asset Line Items
Regulatory Assets, Total	21,800,000	21,500,000
The Connecticut Light and Power Company [Member] | Deferred Environmental Remediation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Public Service Company of New Hampshire [Member] | Deferred Environmental Remediation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	9,700,000	9,700,000
Western Massachusetts Electric Company [Member] | Deferred Environmental Remediation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Deferred Environmental Remediation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	39,000,000	36,800,000
The Connecticut Light and Power Company [Member] | Deferred Operation and Maintenance Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	5,900,000	29,500,000
Public Service Company of New Hampshire [Member] | Deferred Operation and Maintenance Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Western Massachusetts Electric Company [Member] | Deferred Operation and Maintenance Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Deferred Operation and Maintenance Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	5,900,000	29,500,000
The Connecticut Light and Power Company [Member] | Other Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	26,600,000	29,000,000
Public Service Company of New Hampshire [Member] | Other Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	23,900,000	19,600,000
Western Massachusetts Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	9,900,000	13,100,000
Other Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	79,500,000	81,500,000
The Connecticut Light and Power Company [Member]
Regulatory Asset Line Items
Regulatory Assets Current	170,338,000	157,530,000
Regulatory Assets Long Term	1,755,094,000	1,721,416,000
Regulatory Assets, Total	1,925,400,000	1,878,900,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount Of Regulatory Costs Not Yet Approved	900,000	600,000
Major Storm Costs Incurred	Major Storms: On August 28, 2011, Tropical Storm Irene caused extensive damage to the NU overhead electric distribution system, particularly at CL&P. The estimated cost of restoration that was deferred for future recovery from customers and recorded as a regulatory asset as of September 30, 2011 for CL&P totaled approximately $92 million. The estimated cost of restoration is subject to change as additional cost information becomes available. CL&P is currently allowed to collect from customers $3 million per year for major storm costs. CL&P will seek recovery of the Tropical Storm Irene deferred storm costs through the appropriate regulatory recovery process. Management believes CL&P will be allowed to recover these deferred storm costs. On October 29, 2011, a snowstorm delivered high winds and heavy snowfall across the NU service territory, causing significant damage to NU’s distribution and transmission systems. Approximately 1.2 million NU customers were without power at the peak of the outages, with approximately 830,000 of those customers in Connecticut, approximately 240,000 of those customers in New Hampshire, and approximately 140,000 of those customers in Massachusetts. In terms of customer outages, this was the largest in CL&P’s history, surpassing Tropical Storm Irene, the third largest in PSNH’s history and the largest in WMECO's history. As a result of the magnitude of the damage, management anticipates the costs of restoring service will approximate or exceed those of Tropical Storm Irene. As management expects such costs to meet the criteria for recovery in Connecticut, Massachusetts, and New Hampshire, CL&P, WMECO and PSNH will seek recovery of these anticipated deferred storm costs through their applicable regulatory recovery process and as a result, these respective costs will be deferred as either a long-term regulatory asset or long-term other deferred debit at CL&P, PSNH and WMECO.
Regulatory Liabilities Line Items
Regulatory Liability Current	111,895,000	75,716,000
Regulatory Liabilities Long-Term	145,876,000	206,394,000
Regulatory Liabilities, Total	257,800,000	282,100,000
The Connecticut Light and Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	69,600,000	78,600,000
The Connecticut Light and Power Company [Member] | Regulatory Liabilities Offsetting Derivative Assets [[Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	0	38,100,000
The Connecticut Light and Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	107,600,000	79,400,000
The Connecticut Light and Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	57,900,000	56,500,000
The Connecticut Light and Power Company [Member] | Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	14,600,000	14,600,000
The Connecticut Light and Power Company [Member] | Wholesale Transmission Overcollections [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	2,000,000	13,700,000
The Connecticut Light and Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	6,100,000	1,200,000
Public Service Company of New Hampshire [Member]
Regulatory Asset Line Items
Regulatory Assets Current	20,679,000	39,215,000
Regulatory Assets Long Term	358,065,000	395,203,000
Regulatory Assets, Total	378,800,000	434,400,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount Of Regulatory Costs Not Yet Approved	6,300,000	26,500,000
Regulatory Liabilities Line Items
Regulatory Liability Current	25,614,000	8,365,000
Regulatory Liabilities Long-Term	56,551,000	58,631,000
Regulatory Liabilities, Total	82,200,000	67,000,000
Public Service Company of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	55,200,000	57,300,000
Public Service Company of New Hampshire [Member] | Regulatory Liabilities Offsetting Derivative Assets [[Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	0	0
Public Service Company of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	22,500,000	6,600,000
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	0	0
Public Service Company of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	4,500,000	3,100,000
Western Massachusetts Electric Company [Member]
Regulatory Asset Line Items
Regulatory Assets Current	24,248,000	19,531,000
Regulatory Assets Long Term	182,336,000	207,584,000
Regulatory Assets, Total	206,500,000	227,100,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount Of Regulatory Costs Not Yet Approved	1,100,000	1,900,000
Regulatory Liabilities Line Items
Regulatory Liability Current	32,617,000	7,959,000
Regulatory Liabilities Long-Term	17,021,000	15,048,000
Regulatory Liabilities, Total	49,600,000	23,000,000
Western Massachusetts Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	7,600,000	9,500,000
Western Massachusetts Electric Company [Member] | Regulatory Liabilities Offsetting Derivative Assets [[Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	0	0
Western Massachusetts Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	20,900,000	4,800,000
Western Massachusetts Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	9,300,000	5,600,000
Western Massachusetts Electric Company [Member] | WMECO Provision For Rate Refunds Member
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	1,300,000	2,000,000
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	8,500,000	0
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	2,000,000	1,100,000
Cost Of Removal [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	180,700,000	194,800,000
Regulatory Liabilities Offsetting Derivative Assets [[Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	0	38,100,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	152,400,000	95,100,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	67,200,000	62,100,000
Pension Liability Yankee Gas Acquisition [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	10,600,000	12,500,000
Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	14,600,000	14,600,000
Wholesale Transmission Overcollections [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	10,500,000	13,700,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	$ 14,800,000	$ 8,200,000