REGULATORY ACCOUNTING (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Regulatory Asset [Line Items]
Regulatory Assets Current	$ 577,010,000	$ 705,025,000
Regulatory Assets Long Term	4,817,305,000	5,132,411,000
Regulatory Assets, Total	5,394,300,000	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 $81.8 million ($3.7 million for CL&P, $27.3 million for NSTAR Electric, $37.4 million for PSNH, and $7.4 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 June 30, 2013 and December 31, 2012, respectively, which were included in Other Long-Term Assets on the 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.
Regulatory Liabilities [Line Items]
Regulatory Liability Current	216,422,000	134,115,000
Regulatory Liabilities Long-Term	511,737,000	540,162,000
Regulatory Liabilities, Total	728,100,000	674,300,000
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	437,100,000	440,800,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	164,900,000	95,100,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	68,500,000	70,000,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	57,600,000	68,400,000
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	173,484,000	185,858,000
Regulatory Assets Long Term	2,055,333,000	2,158,363,000
Regulatory Assets, Total	2,228,800,000	2,344,300,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	64,007,000	32,119,000
Regulatory Liabilities Long-Term	99,863,000	124,319,000
Regulatory Liabilities, Total	163,900,000	156,400,000
The Connecticut Light And Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	34,700,000	44,200,000
The Connecticut Light And Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	66,000,000	39,100,000
The Connecticut Light And Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	55,200,000	56,600,000
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	8,000,000	16,500,000
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	270,813,000	347,081,000
Regulatory Assets Long Term	1,461,875,000	1,444,870,000
Regulatory Assets, Total	1,732,700,000	1,792,000,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	74,435,000	47,539,000
Regulatory Liabilities Long-Term	249,862,000	244,224,000
Regulatory Liabilities, Total	324,300,000	291,700,000
NSTAR Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	246,000,000	240,300,000
NSTAR Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	42,900,000	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	31,400,000	32,900,000
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	55,836,000	62,882,000
Regulatory Assets Long Term	311,442,000	351,059,000
Regulatory Assets, Total	367,200,000	414,000,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	21,913,000	23,002,000
Regulatory Liabilities Long-Term	51,039,000	52,418,000
Regulatory Liabilities, Total	72,900,000	75,400,000
Public Service Company Of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	50,300,000	51,200,000
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	17,600,000	20,400,000
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	5,000,000	3,800,000
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets Current	45,036,000	42,370,000
Regulatory Assets Long Term	192,231,000	221,752,000
Regulatory Assets, Total	237,200,000	264,200,000
Regulatory Liabilities [Line Items]
Regulatory Liability Current	19,885,000	21,037,000
Regulatory Liabilities Long-Term	11,058,000	9,686,000
Regulatory Liabilities, Total	31,000,000	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	19,200,000	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	2,500,000	2,400,000
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	2,256,700,000	2,452,100,000
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	525,600,000	563,200,000
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	783,700,000	781,200,000
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	205,900,000	223,700,000
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	107,100,000	116,000,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	793,300,000	885,600,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	775,800,000	866,200,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	13,100,000	14,900,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,500,000	0
Regulatory Assets Offsetting Derivative Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	700,000	3,000,000
Goodwill [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	527,500,000	537,600,000
Goodwill [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	452,900,000	461,500,000
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	636,300,000	547,700,000
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	445,700,000	413,900,000
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	119,100,000	55,800,000
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	30,100,000	34,500,000
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	41,400,000	43,500,000
Deferred Income Tax Charges Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	529,800,000	516,200,000
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	379,800,000	367,500,000
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	46,200,000	47,100,000
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	36,600,000	36,200,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,400,000	31,000,000
Securitized Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	97,100,000	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	97,100,000	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	126,400,000	217,600,000
Unrecovered Contractual Obligations [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	20,100,000	64,000,000
Unrecovered Contractual Obligations [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	6,700,000	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	4,800,000	14,900,000
Power Contracts Buy Out Agreements [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	81,700,000	92,900,000
Power Contracts Buy Out Agreements [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	75,500,000	85,900,000
Power Contracts Buy Out Agreements [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	6,200,000	7,000,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	194,500,000	190,100,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	21,500,000	12,200,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	100,800,000	71,400,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	44,000,000	49,300,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	26,400,000	31,900,000
Asset Retirement Obligation Costs Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	91,500,000	88,800,000
Asset Retirement Obligation Costs Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	30,500,000	29,400,000
Asset Retirement Obligation Costs Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	30,200,000	29,400,000
Asset Retirement Obligation Costs Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	14,500,000	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	59,500,000	76,200,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	29,800,000	27,900,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	7,400,000	16,900,000
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	28,400,000	29,400,000
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 21,800,000	$ 12,600,000