REGULATORY ACCOUNTING (Details) (USD $)	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 3,806,700,000	$ 4,294,500,000
Regulatory Assets Current	467,156,000	535,791,000
Regulatory Assets Long Term	3,339,457,000	3,758,694,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Description of Regulatory Costs Not yet Approved	Regulatory Costs in Other Long-Term Assets: The Regulated companies had $64.5 million ($3.4 million for CL&P, $33.9 million for NSTAR Electric, and $12 million for WMECO) and $65.1 million ($7.3 million for CL&P, $33.4 million for NSTAR Electric, and $10.1 million for WMECO) of additional regulatory costs as of June 30, 2014 and December 31, 2013, respectively, that were included in Other Long-Term Assets on the balance sheets. These amounts represent incurred costs for which recovery has not yet been specifically approved by the applicable regulatory agency. However, based on regulatory policies or past precedent on similar costs, management believes it is probable that these costs will ultimately be approved and recovered from customers in rates.
Description Of Regulatory Costs Recently Approved	Storm Restoration Costs: On March 12, 2014, the PURA approved recovery of $365 million of deferred storm restoration costs associated with five major storms that occurred in 2011 and 2012. CL&P will recover the $365 million with carrying charges in its distribution rates over a six-year period beginning December 1, 2014. On June 17, 2014, the PURA ordered CL&P to use the DOE Phase II Damages proceeds of $65.4 million to offset the $365 million in 2011 and 2012 deferred storm restoration costs, which are reflected in the deferred storm restoration costs regulatory asset. For further information on the DOE Phase II Damages proceeds received from the Yankee Companies, see Note 8C, "Commitments and Contingencies - Contractual Obligations - Yankee Companies," to the financial statements.
Description of Regulatory Assets and Liabilities	As a result of two FERC orders issued on June 19, 2014 in the pending base ROE complaint proceedings described in Note 8E, "Commitments and Contingencies – FERC Base ROE Complaints," in the second quarter of 2014, the Company recorded a series of reserves at its electric subsidiaries to recognize the potential financial impact of these rulings. The aggregate pre-tax charge totaled $54.7 million at NU, which represented reserves of $31.4 million at CL&P, $10.3 million at NSTAR Electric, $3.8 million at PSNH and $9.2 million at WMECO. As of June 30, 2014, the cumulative reserves totaled $79.3 million at NU, $44.7 million at CL&P, $16.2 million at NSTAR Electric, $6.2 million at PSNH and $12.2 million at WMECO. As of December 31, 2013, as a result of the FERC ALJ initial decision in the third quarter of 2013, the Company had an aggregate pre-tax reserve of $24.6 million at NU, which represented reserves of $13.3 million at CL&P, $5.9 million at NSTAR Electric, $2.4 million at PSNH and $3 million at WMECO. These reserves were recorded in each electric subsidiary's respective transmission regulatory tracker mechanism and as a reduction of operating revenues. As a result of awards issued to the Yankee Companies for spent nuclear fuel lawsuits against the DOE described in Note 8C, "Commitments and Contingencies - Contractual Obligations - Yankee Companies," the Yankee Companies returned the DOE Phase II Damages proceeds to the member companies, including CL&P, NSTAR Electric, PSNH, and WMECO, for the benefit of their respective customers, effective June 1, 2014. CL&P’s refund obligation to customers of $65.4 million was recorded as an offset to the deferred storm restoration costs regulatory asset, as directed by PURA. NSTAR Electric’s, PSNH’s and WMECO’s refund obligation to customers of $29.1 million, $13.1 million and $18.1 million, respectively, was recorded as a regulatory liability in each electric subsidiary’s respective regulatory tracker mechanisms.
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	863,900,000	707,300,000
Regulatory Liability Current	359,921,000	204,278,000
Regulatory Liabilities Long-Term	503,955,000	502,984,000
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	435,300,000	435,100,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	305,200,000	151,200,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	67,400,000	68,100,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	56,000,000	52,900,000
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,502,500,000	1,814,000,000
Regulatory Assets Current	109,951,000	150,943,000
Regulatory Assets Long Term	1,392,529,000	1,663,147,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	230,200,000	187,800,000
Regulatory Liability Current	143,457,000	93,961,000
Regulatory Liabilities Long-Term	86,677,000	93,757,000
The Connecticut Light And Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	22,800,000	29,100,000
The Connecticut Light And Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	143,200,000	95,600,000
The Connecticut Light And Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	54,200,000	54,700,000
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	10,000,000	8,400,000
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,199,600,000	1,439,300,000
Regulatory Assets Current	178,640,000	204,144,000
Regulatory Assets Long Term	1,020,990,000	1,235,156,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	349,700,000	307,100,000
Regulatory Liability Current	89,161,000	53,958,000
Regulatory Liabilities Long-Term	260,480,000	253,108,000
NSTAR Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	255,700,000	250,000,000
NSTAR Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	60,200,000	21,900,000
NSTAR Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,000,000	4,100,000
NSTAR Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	29,800,000	31,100,000
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	282,900,000	311,500,000
Regulatory Assets Current	95,270,000	92,194,000
Regulatory Assets Long Term	187,592,000	219,346,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	87,400,000	72,300,000
Regulatory Liability Current	36,627,000	20,643,000
Regulatory Liabilities Long-Term	50,843,000	51,723,000
Public Service Company Of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	48,700,000	49,700,000
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	34,800,000	21,600,000
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	3,900,000	1,000,000
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	156,600,000	189,100,000
Regulatory Assets Current	36,251,000	43,024,000
Regulatory Assets Long Term	120,303,000	146,088,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	55,000,000	33,800,000
Regulatory Liability Current	44,745,000	19,858,000
Regulatory Liabilities Long-Term	10,317,000	13,873,000
Western Massachusetts Electric Company [Member] | Cost Of Removal [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	45,100,000	21,100,000
Western Massachusetts Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	9,200,000	9,300,000
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	700,000	3,400,000
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,146,700,000	1,240,200,000
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	251,900,000	297,700,000
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	323,400,000	496,700,000
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	81,600,000	100,600,000
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	46,200,000	57,300,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	431,400,000	638,000,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	424,600,000	630,400,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	6,300,000	7,700,000
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	0	0
Deferred Income Tax Charges Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	631,800,000	626,200,000
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	426,200,000	415,500,000
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	81,400,000	84,000,000
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	38,000,000	40,300,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	40,800,000	43,700,000
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	503,900,000	589,600,000
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	328,000,000	397,800,000
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	107,200,000	109,300,000
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	34,700,000	43,700,000
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	34,000,000	38,800,000
Goodwill Regulatory Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	515,700,000	525,900,000
Goodwill Regulatory Asset [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	442,800,000	451,500,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	275,100,000	323,400,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	8,100,000	8,000,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	131,800,000	169,500,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	87,900,000	83,300,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	20,700,000	32,600,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	128,400,000	154,200,000
Power Contracts Buy Out Agreements [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	56,700,000	70,200,000
Power Contracts Buy Out Agreements [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	52,000,000	64,700,000
Power Contracts Buy Out Agreements [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	4,700,000	5,500,000
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	117,000,000	126,800,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	63,700,000	64,600,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	54,700,000	55,900,000
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	36,000,000	38,100,000
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 14,900,000	$ 16,700,000