REGULATORY ACCOUNTING (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Regulatory Asset [Line Items]
Regulatory Assets Current	$ 474,198,000	$ 705,025,000
Regulatory Assets Long Term	4,882,381,000	5,132,411,000
Regulatory Assets, Total	5,356,600,000	5,837,400,000
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 $95.1 million ($3.4 million for CL&P, $31.3 million for NSTAR Electric, $37.3 million for PSNH, and $7.9 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 additional regulatory costs as of September 30, 2013 and December 31, 2012, respectively, which were included in Other Long-Term Assets on the balance sheets. These amounts represent incurred costs for which specific recovery has not yet been approved by the applicable regulatory agency. Management believes it is probable that these costs will ultimately be approved and recovered from customers.
Regulatory Liabilities [Line Items]
Regulatory Liability Current	224,416,000	134,115,000
Regulatory Liabilities Long-Term	520,732,000	540,162,000
Regulatory Liabilities, Total	745,100,000	674,300,000
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	434,300,000	440,800,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	168,600,000	95,100,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	68,300,000	70,000,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	73,900,000	68,400,000
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	147,076,000	185,858,000
Regulatory Assets Long Term	2,021,974,000	2,158,363,000
Regulatory Assets, Total	2,169,100,000	2,344,300,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	81,988,000	32,119,000
Regulatory Liabilities Long-Term	107,964,000	124,319,000
Regulatory Liabilities, Total	190,000,000	156,400,000
The Connecticut Light And Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	31,200,000	44,200,000
The Connecticut Light And Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	73,200,000	39,100,000
The Connecticut Light And Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	55,000,000	56,600,000
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	30,600,000	16,500,000
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	189,754,000	347,081,000
Regulatory Assets Long Term	1,538,222,000	1,444,870,000
Regulatory Assets, Total	1,728,000,000	1,792,000,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	82,521,000	47,539,000
Regulatory Liabilities Long-Term	251,005,000	244,224,000
Regulatory Liabilities, Total	333,500,000	291,700,000
NSTAR Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	247,200,000	240,300,000
NSTAR Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	51,000,000	14,400,000
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	31,300,000	32,900,000
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	67,716,000	62,882,000
Regulatory Assets Long Term	301,368,000	351,059,000
Regulatory Assets, Total	369,100,000	414,000,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	23,394,000	23,002,000
Regulatory Liabilities Long-Term	52,867,000	52,418,000
Regulatory Liabilities, Total	76,300,000	75,400,000
Public Service Company Of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	49,800,000	51,200,000
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	10,700,000	20,400,000
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	15,800,000	3,800,000
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	37,854,000	42,370,000
Regulatory Assets Long Term	194,744,000	221,752,000
Regulatory Assets, Total	232,600,000	264,200,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	22,400,000	21,037,000
Regulatory Liabilities Long-Term	11,914,000	9,686,000
Regulatory Liabilities, Total	34,300,000	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	22,100,000	19,000,000
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] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,900,000	2,400,000
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	2,256,000,000	2,452,100,000
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	509,300,000	563,200,000
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	824,300,000	781,200,000
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	199,600,000	223,700,000
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	103,800,000	116,000,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	770,300,000	885,600,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	755,300,000	866,200,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	11,600,000	14,900,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	300,000	0
Regulatory Assets Offsetting Derivative Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	3,000,000
Goodwill Regulatory Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	531,100,000	537,600,000
Goodwill Regulatory Asset [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	455,900,000	461,500,000
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	621,000,000	547,700,000
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	439,400,000	413,900,000
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	114,000,000	55,800,000
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	27,900,000	34,500,000
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	39,700,000	43,500,000
Deferred Income Tax Charges Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	587,500,000	516,200,000
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	385,300,000	367,500,000
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	84,800,000	47,100,000
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	36,500,000	36,200,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	42,100,000	31,000,000
Securitized Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	37,400,000	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	37,400,000	205,100,000
Securitized Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	19,700,000
Securitized Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	7,800,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	170,900,000	217,600,000
Unrecovered Contractual Obligations [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	20,000,000	64,000,000
Unrecovered Contractual Obligations [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	6,400,000	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	4,600,000	14,900,000
Power Contracts Buy Out Agreements [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	76,000,000	92,900,000
Power Contracts Buy Out Agreements [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	70,100,000	85,900,000
Power Contracts Buy Out Agreements [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	5,900,000	7,000,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	163,300,000	190,100,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	12,200,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	83,600,000	71,400,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	52,500,000	49,300,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	21,500,000	31,900,000
Asset Retirement Obligation Costs Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	93,000,000	88,800,000
Asset Retirement Obligation Costs Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,100,000	29,400,000
Asset Retirement Obligation Costs Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	30,700,000	29,400,000
Asset Retirement Obligation Costs Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	14,700,000	14,200,000
Asset Retirement Obligation Costs Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	3,700,000	3,500,000
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	50,100,000	76,200,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	28,700,000	27,900,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	9,200,000	16,900,000
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,700,000	29,400,000
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 17,200,000	$ 12,600,000