REGULATORY ACCOUNTING (Details) (USD $)	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Regulatory Asset [Line Items]
Regulatory Assets Current	$ 647,615,000	$ 255,144,000
Regulatory Assets Long Term	5,008,034,000	3,267,710,000
Regulatory Assets, Total	5,655,600,000	3,522,800,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Description of Regulatory Costs Not yet Approved	Regulatory Costs Not Yet Approved: Additionally, the Regulated companies had $57 million ($4.1 million for CL&P, $24.4 million for NSTAR Electric, $23.4 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 September 30, 2012 and December 31, 2011, respectively, which were included in Other Long-Term Assets on the accompanying unaudited condensed consolidated balance sheets. For comparative purposes, NSTAR Electric had $9.5 million of such regulatory costs as of December 31, 2011. These amounts represent incurred costs that have not yet been approved for recovery by the applicable regulatory agency. Management believes it is probable that these costs will be recovered in future cost-of-service regulated rates.
Regulatory Liabilities [Line Items]
Regulatory Liability Current	226,606,000	167,844,000
Regulatory Liabilities Long-Term	561,555,000	266,145,000
Regulatory Liabilities, Total	788,200,000	433,900,000
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	447,200,000	172,200,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	171,200,000	139,100,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	70,200,000	67,000,000
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	1,600,000	9,600,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	82,600,000	30,600,000
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	186,594,000	170,197,000
Regulatory Assets Long Term	1,998,426,000	2,103,830,000
Regulatory Assets, Total	2,185,000,000	2,274,000,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Major Storm Costs Incurred, Tropical Storm Irene And October 2011 Snowstorm	Storm Costs: On August 1, 2012, PURA issued a final decision in the investigation of CL&P’s performance related to both Tropical Storm Irene and the October 2011 snowstorm. The decision identified certain penalties that could be imposed on CL&P during its next rate case, including a reduction in allowed regulatory ROE and the disallowance of certain deferred storm restoration costs. However, PURA will consider and weigh the extent to which CL&P has taken steps in its restructuring of storm management and the establishment of new practices for execution in future storm response in determining any potential penalties. At this time, management cannot estimate the impact on CL&P’s financial position, results of operations or cash flows. CL&P continues to believe that its response to these events was prudent, is consistent with industry standards, and that it is probable it will be able to recover its deferred costs. As of September 30, 2012, CL&P had recorded total deferred storm costs relating to Tropical Storm Irene and the October 2011 snowstorm of $284.9 million. The storm cost deferral regulatory asset balance is net of a reserve of $40 million recorded in connection with the Connecticut settlement agreement. See Note 2, "Merger of NU and NSTAR," for further information.
Regulatory Liabilities [Line Items]
Regulatory Liability Current	90,208,000	108,291,000
Regulatory Liabilities Long-Term	130,116,000	139,307,000
Regulatory Liabilities, Total	220,300,000	247,600,000
The Connecticut Light And Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	47,100,000	63,800,000
The Connecticut Light And Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	86,300,000	94,400,000
The Connecticut Light And Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	56,800,000	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	3,000,000	4,500,000
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	11,700,000	11,800,000
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	337,575,000	323,871,000
Regulatory Assets Long Term	1,510,087,000	1,680,595,000
Regulatory Assets, Total	1,847,700,000	2,004,500,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	84,476,000	41,579,000
Regulatory Liabilities Long-Term	256,542,000	239,858,000
Regulatory Liabilities, Total	341,000,000	281,500,000
NSTAR Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	242,200,000	235,800,000
NSTAR Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	43,700,000	11,700,000
NSTAR Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,100,000	4,300,000
NSTAR Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	51,000,000	29,700,000
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	34,767,000	34,178,000
Regulatory Assets Long Term	322,896,000	393,941,000
Regulatory Assets, Total	357,700,000	428,100,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	18,373,000	24,500,000
Regulatory Liabilities Long-Term	53,418,000	54,415,000
Regulatory Liabilities, Total	71,800,000	78,900,000
Public Service Company Of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	52,200,000	53,200,000
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	16,300,000	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] | Wholesale Transmission Overcollections [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	0	2,600,000
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	3,300,000	5,800,000
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	34,839,000	35,520,000
Regulatory Assets Long Term	213,928,000	233,247,000
Regulatory Assets, Total	248,700,000	268,700,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	22,479,000	33,056,000
Regulatory Liabilities Long-Term	9,708,000	16,597,000
Regulatory Liabilities, Total	32,200,000	49,700,000
Western Massachusetts Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	0	7,200,000
Western Massachusetts Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	18,200,000	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] | Wholesale Transmission Overcollections [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,600,000	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	2,374,300,000	1,360,500,000
Deferred Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	516,700,000	572,800,000
Deferred Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	811,900,000	813,700,000
Deferred Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	178,100,000	200,000,000
Deferred Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	107,600,000	118,900,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	908,900,000	939,600,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	891,000,000	932,000,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	13,800,000	3,400,000
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	3,700,000	7,300,000
Goodwill [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	542,600,000
Goodwill [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	465,900,000	478,900,000
Deferred Income Tax Charges Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	511,200,000	425,400,000
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	364,100,000	339,600,000
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	47,500,000	48,800,000
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	36,000,000	38,000,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	30,200,000	17,800,000
Storm Cost Deferrals [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	369,500,000	356,000,000
Storm Cost Deferrals [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	255,700,000	268,300,000
Storm Cost Deferrals [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	36,900,000	30,600,000
Storm Cost Deferrals [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	36,700,000	44,000,000
Storm Cost Deferrals [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	40,200,000	43,700,000
Securitized Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	293,000,000	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] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	246,500,000	368,500,000
Securitized Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	34,200,000	76,400,000
Securitized Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	12,300,000	25,400,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	215,800,000	100,900,000
Unrecovered Contractual Obligations [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	68,200,000	80,900,000
Unrecovered Contractual Obligations [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	24,800,000	30,800,000
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	16,200,000	20,000,000
Power Contracts Buy Out Agreements [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	99,300,000	8,600,000
Power Contracts Buy Out Agreements [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	91,900,000	109,500,000
Power Contracts Buy Out Agreements [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	7,400,000	8,600,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	132,100,000	45,900,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	18,900,000	5,500,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	55,500,000	61,100,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	21,900,000	11,900,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	23,100,000	22,100,000
Asset Retirement Obligation Costs Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	83,800,000	47,500,000
Asset Retirement Obligation Costs Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	29,700,000	27,900,000
Asset Retirement Obligation Costs Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	25,800,000	24,500,000
Asset Retirement Obligation Costs Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	14,000,000	13,500,000
Asset Retirement Obligation Costs Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	3,500,000	3,200,000
Loss On Reacquired Debt Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	39,500,000	24,500,000
Loss On Reacquired Debt Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	13,400,000	13,900,000
Loss On Reacquired Debt Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	16,300,000	18,200,000
Loss On Reacquired Debt Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	8,300,000	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	57,500,000	38,500,000
Deferred Environmental Remediation Costs Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	9,900,000	9,700,000
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	28,100,000	73,600,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	27,300,000	33,100,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	10,900,000	16,500,000
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	11,200,000	17,000,000
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 11,600,000	$ 10,000,000