REGULATORY ACCOUNTING (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Regulatory Asset [Line Items]
Regulatory Assets Current	$ 625,542,000	$ 705,025,000
Regulatory Assets Long Term	5,015,108,000	5,132,411,000
Regulatory Assets, Total	5,640,600,000	5,837,400,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Description of Regulatory Costs Not yet Approved	Regulatory Costs Not Yet Approved: The Regulated companies had $78.8 million ($3.7 million for CL&P, $26.3 million for NSTAR Electric, $37.2 million for PSNH, and $6 million for WMECO) and $69.9 million ($3.9 million for CL&P, $25.4 million for NSTAR Electric, $35.7 million for PSNH, and $1.4 million for WMECO) of regulatory costs as of March 31, 2013 and December 31, 2012, respectively, which were included in Other Long-Term Assets on the consolidated balance sheets. 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 ultimately be approved and recovered from customers.
Major Storm Costs Incurred	Storm Restoration Costs: The storm restoration cost deferrals relate to costs incurred at CL&P, NSTAR Electric, PSNH and WMECO that each company expects to collect from customers. A storm must meet certain criteria specific to each state and utility company to be declared a major storm. Once a storm is declared major, all qualifying expenses incurred during storm restoration efforts, if deemed prudent, are deferred and recovered from customers in future periods. In Connecticut, qualifying storm restoration costs must exceed $5 million for a storm to be declared a major storm. In Massachusetts, qualifying storm restoration costs must exceed $1 million for NSTAR Electric and $300,000 for WMECO and an emergency response plan must be initiated for a storm to be declared a major storm. In New Hampshire, (1) at least 10 percent of customers must be without power with at least 200 concurrent locations requiring repairs (trouble spots), or (2) at least 300 concurrent trouble spots must be reported for a storm to be declared a major storm. In addition, PSNH is permitted to defer pre-staging costs for storms that may not ultimately qualify as a major storm. On February 8, 2013, a blizzard caused damage to the electric delivery systems of CL&P and NSTAR Electric. This blizzard resulted in estimated deferred storm restoration costs of approximately $13 million at CL&P and approximately $80 million at NSTAR Electric. On January 31, 2013, a major storm caused damage to CL&P's electric delivery system, which resulted in estimated deferred storm restoration costs of approximately $19 million. Management believes these storm restoration costs meet the criteria for specific cost recovery in Connecticut and Massachusetts and, as a result, are probable of recovery. Therefore, management does not expect these storms to have a material impact on CL&P's or NSTAR Electric’s results of operations. CL&P and NSTAR Electric will seek recovery of these deferred storm restoration costs through their applicable regulatory recovery process.
Regulatory Liabilities [Line Items]
Regulatory Liability Current	191,707,000	134,115,000
Regulatory Liabilities Long-Term	531,546,000	540,162,000
Regulatory Liabilities, Total	723,200,000	674,300,000
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	441,300,000	440,800,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	147,000,000	95,100,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	68,900,000	70,000,000
Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	15,400,000	15,400,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	50,600,000	53,000,000
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	172,883,000	185,858,000
Regulatory Assets Long Term	2,141,137,000	2,158,363,000
Regulatory Assets, Total	2,314,000,000	2,344,300,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	42,324,000	32,119,000
Regulatory Liabilities Long-Term	110,688,000	124,319,000
Regulatory Liabilities, Total	153,000,000	156,400,000
The Connecticut Light And Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	39,300,000	44,200,000
The Connecticut Light And Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	39,100,000	39,100,000
The Connecticut Light And Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	55,500,000	56,600,000
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	2,300,000	0
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	1,400,000	1,100,000
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	309,559,000	347,081,000
Regulatory Assets Long Term	1,489,317,000	1,444,870,000
Regulatory Assets, Total	1,798,900,000	1,792,000,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	67,277,000	47,539,000
Regulatory Liabilities Long-Term	248,376,000	244,224,000
Regulatory Liabilities, Total	315,700,000	291,700,000
NSTAR Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	244,500,000	240,300,000
NSTAR Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	37,300,000	14,400,000
NSTAR Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,100,000	4,100,000
NSTAR Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	29,800,000	32,900,000
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	62,184,000	62,882,000
Regulatory Assets Long Term	329,314,000	351,059,000
Regulatory Assets, Total	391,500,000	414,000,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	18,132,000	23,002,000
Regulatory Liabilities Long-Term	52,473,000	52,418,000
Regulatory Liabilities, Total	70,600,000	75,400,000
Public Service Company Of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	51,300,000	51,200,000
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	13,800,000	20,400,000
Public Service Company Of New Hampshire [Member] | Wholesale Transmission Overcollections [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	1,100,000	0
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,400,000	3,800,000
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	43,534,000	42,370,000
Regulatory Assets Long Term	207,901,000	221,752,000
Regulatory Assets, Total	251,400,000	264,200,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	15,638,000	21,037,000
Regulatory Liabilities Long-Term	9,736,000	9,686,000
Regulatory Liabilities, Total	25,300,000	30,700,000
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	12,500,000	13,700,000
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	900,000	5,300,000
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,600,000	2,400,000
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	2,390,400,000	2,452,100,000
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	546,900,000	563,200,000
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	769,000,000	781,200,000
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	217,200,000	223,700,000
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	112,600,000	116,000,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	835,000,000	885,600,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	819,600,000	866,200,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	13,600,000	14,900,000
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	1,800,000	3,000,000
Goodwill [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	532,500,000	537,600,000
Goodwill [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	457,200,000	461,500,000
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	656,100,000	547,700,000
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	448,700,000	413,900,000
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	132,700,000	55,800,000
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	32,300,000	34,500,000
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	42,400,000	43,500,000
Deferred Income Tax Charges Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	521,700,000	516,200,000
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	371,700,000	367,500,000
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	46,700,000	47,100,000
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	37,300,000	36,200,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,100,000	31,000,000
Securitized Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	157,600,000	232,600,000
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	149,500,000	205,100,000
Securitized Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	5,000,000	19,700,000
Securitized Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	3,100,000	7,800,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	124,900,000	217,600,000
Unrecovered Contractual Obligations [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	59,700,000	64,000,000
Unrecovered Contractual Obligations [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	20,900,000	22,800,000
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	13,700,000	14,900,000
Power Contracts Buy Out Agreements [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	87,300,000	92,900,000
Power Contracts Buy Out Agreements [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	80,700,000	85,900,000
Power Contracts Buy Out Agreements [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	6,600,000	7,000,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	177,200,000	190,100,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	10,100,000	12,200,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	84,500,000	71,400,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	49,700,000	49,300,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,300,000	31,900,000
Asset Retirement Obligation Costs Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	90,200,000	88,800,000
Asset Retirement Obligation Costs Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	30,000,000	29,400,000
Asset Retirement Obligation Costs Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	29,800,000	29,400,000
Asset Retirement Obligation Costs Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	14,300,000	14,200,000
Asset Retirement Obligation Costs Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	3,600,000	3,500,000
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	67,700,000	76,200,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	27,300,000	27,900,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	14,300,000	16,900,000
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	29,100,000	29,400,000
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 11,800,000	$ 12,600,000