REGULATORY ACCOUNTING (Details) (USD $)	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Regulatory Asset Line Items
Regulatory Assets Current	$ 242,137,000	$ 238,699,000
Regulatory Assets Long Term	2,656,093,000	2,756,580,000
Regulatory Assets, Total	2,898,200,000	2,995,300,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount Of Regulatory Costs Not Yet Approved	4,400,000	37,500,000
Major Storm Costs Incurred	Major Storms: On June 1, 2011, a series of severe thunderstorms with high winds, including a tornado, struck portions of WMECO’s service territory. The cost of restoring power, including rebuilding certain overhead electric distribution equipment and services, that was deferred for future recovery and recorded as a regulatory asset as of June 30, 2011 totaled $3.2 million. On June 9, 2011, another series of severe thunderstorms with high winds struck CL&P, PSNH and WMECO's service territories. The cost of restoration that was deferred for future recovery from customers and recorded as regulatory assets as of June 30, 2011 for CL&P and WMECO totaled $7.9 million and $1.2 million, respectively.
Regulatory Liabilities Line Items
Regulatory Liability Current	152,956,000	99,403,000
Regulatory Liabilities Long-Term	273,909,000	339,655,000
Regulatory Liabilities, Total	426,900,000	439,100,000
The Connecticut Light and Power Company [Member] | Deferred Benefit Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	445,600,000	471,800,000
Public Service Company of New Hampshire [Member] | Deferred Benefit Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	145,900,000	152,600,000
Western Massachusetts Electric Company [Member] | Deferred Benefit Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	91,200,000	96,000,000
Deferred Benefit Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	1,036,100,000	1,094,200,000
The Connecticut Light and Power Company [Member] | Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	859,800,000	846,200,000
Public Service Company of New Hampshire [Member] | Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	6,700,000	12,800,000
Western Massachusetts Electric Company [Member] | Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	866,900,000	859,700,000
The Connecticut Light and Power Company [Member] | Securitized Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Public Service Company of New Hampshire [Member] | Securitized Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	103,600,000	129,800,000
Western Massachusetts Electric Company [Member] | Securitized Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	33,700,000	41,900,000
Securitized Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	137,300,000	171,700,000
The Connecticut Light and Power Company [Member] | Deferred Income Tax Charges Member
Regulatory Asset Line Items
Regulatory Assets, Total	340,400,000	328,900,000
Public Service Company of New Hampshire [Member] | Deferred Income Tax Charges Member
Regulatory Asset Line Items
Regulatory Assets, Total	35,200,000	31,400,000
Western Massachusetts Electric Company [Member] | Deferred Income Tax Charges Member
Regulatory Asset Line Items
Regulatory Assets, Total	16,600,000	16,800,000
Deferred Income Tax Charges Member
Regulatory Asset Line Items
Regulatory Assets, Total	419,000,000	401,500,000
The Connecticut Light and Power Company [Member] | Unrecovered Contractual Obligations [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	89,600,000	97,900,000
Public Service Company of New Hampshire [Member] | Unrecovered Contractual Obligations [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Western Massachusetts Electric Company [Member] | Unrecovered Contractual Obligations [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	22,800,000	25,300,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	112,400,000	123,200,000
The Connecticut Light and Power Company [Member] | Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	13,900,000	35,500,000
Public Service Company of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	13,900,000	14,700,000
Western Massachusetts Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	16,000,000	15,200,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	48,300,000	70,300,000
The Connecticut Light and Power Company [Member] | Storm Cost Deferrals [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	10,400,000	4,000,000
Public Service Company of New Hampshire [Member] | Storm Cost Deferrals [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	48,500,000	40,700,000
Western Massachusetts Electric Company [Member] | Storm Cost Deferrals [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	18,400,000	15,400,000
Storm Cost Deferrals [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	77,300,000	60,100,000
The Connecticut Light and Power Company [Member] | Asset Retirement Obligation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	26,000,000	24,900,000
Public Service Company of New Hampshire [Member] | Asset Retirement Obligation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	14,900,000	14,700,000
Western Massachusetts Electric Company [Member] | Asset Retirement Obligation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	3,100,000	3,000,000
Asset Retirement Obligation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	46,900,000	45,300,000
The Connecticut Light and Power Company [Member] | Loss On Reacquired Debt Member
Regulatory Asset Line Items
Regulatory Assets, Total	11,100,000	11,200,000
Public Service Company of New Hampshire [Member] | Loss On Reacquired Debt Member
Regulatory Asset Line Items
Regulatory Assets, Total	9,500,000	8,400,000
Western Massachusetts Electric Company [Member] | Loss On Reacquired Debt Member
Regulatory Asset Line Items
Regulatory Assets, Total	300,000	400,000
Loss On Reacquired Debt Member
Regulatory Asset Line Items
Regulatory Assets, Total	22,300,000	21,500,000
The Connecticut Light and Power Company [Member] | Deferred Environmental Remediation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Public Service Company of New Hampshire [Member] | Deferred Environmental Remediation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	9,700,000	9,700,000
Western Massachusetts Electric Company [Member] | Deferred Environmental Remediation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Deferred Environmental Remediation Costs Member
Regulatory Asset Line Items
Regulatory Assets, Total	39,800,000	36,800,000
The Connecticut Light and Power Company [Member] | Deferred Operation and Maintenance Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	7,800,000	29,500,000
Public Service Company of New Hampshire [Member] | Deferred Operation and Maintenance Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Western Massachusetts Electric Company [Member] | Deferred Operation and Maintenance Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	0	0
Deferred Operation and Maintenance Costs [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	7,800,000	29,500,000
The Connecticut Light and Power Company [Member] | Other Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	27,500,000	29,000,000
Public Service Company of New Hampshire [Member] | Other Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	26,400,000	19,600,000
Western Massachusetts Electric Company [Member] | Other Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	9,300,000	13,100,000
Other Regulatory Assets [Member]
Regulatory Asset Line Items
Regulatory Assets, Total	84,100,000	81,500,000
The Connecticut Light and Power Company [Member]
Regulatory Asset Line Items
Regulatory Assets Current	163,917,000	157,530,000
Regulatory Assets Long Term	1,668,232,000	1,721,416,000
Regulatory Assets, Total	1,832,100,000	1,878,900,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount Of Regulatory Costs Not Yet Approved	800,000	600,000
Regulatory Liabilities Line Items
Regulatory Liability Current	102,869,000	75,716,000
Regulatory Liabilities Long-Term	141,635,000	206,394,000
Regulatory Liabilities, Total	244,500,000	282,100,000
The Connecticut Light and Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	72,800,000	78,600,000
The Connecticut Light and Power Company [Member] | Regulatory Liabilities Offsetting Derivative Assets [Member
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	0	38,100,000
The Connecticut Light and Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	84,800,000	79,400,000
The Connecticut Light and Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	58,200,000	56,500,000
The Connecticut Light and Power Company [Member] | Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	14,600,000	14,600,000
The Connecticut Light and Power Company [Member] | Wholesale Transmission Overcollections [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	7,600,000	13,700,000
The Connecticut Light and Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	6,500,000	1,200,000
Public Service Company of New Hampshire [Member]
Regulatory Asset Line Items
Regulatory Assets Current	38,705,000	39,215,000
Regulatory Assets Long Term	375,551,000	395,203,000
Regulatory Assets, Total	414,300,000	434,400,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount Of Regulatory Costs Not Yet Approved		26,500,000
Description Of Regulatory Costs Recently Approved	During June 2011, the NHPUC approved for recovery costs incurred for the February 2010 winter storm restorations and certain costs related to previously recognized tax benefits lost as a result of a provision in the 2010 Healthcare Act that eliminated the tax deductibility of actuarially equivalent Medicare Part D benefits for retirees. Both deferrals were previously recorded in Other Long-Term Assets. As of June 30, 2011, $10.9 million for the February 2010 wind storm costs and $7.2 million for the recovery of the future tax benefits lost as a result of the 2010 Healthcare Act were recorded as Regulatory Assets.
Regulatory Liabilities Line Items
Regulatory Liability Current	22,369,000	8,365,000
Regulatory Liabilities Long-Term	57,104,000	58,631,000
Regulatory Liabilities, Total	79,500,000	67,000,000
Public Service Company of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	55,800,000	57,300,000
Public Service Company of New Hampshire [Member] | Regulatory Liabilities Offsetting Derivative Assets [Member
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	0	0
Public Service Company of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	19,400,000	6,600,000
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] | Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [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	0
Public Service Company of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	4,300,000	3,100,000
Western Massachusetts Electric Company [Member]
Regulatory Asset Line Items
Regulatory Assets Current	19,454,000	19,531,000
Regulatory Assets Long Term	191,939,000	207,584,000
Regulatory Assets, Total	211,400,000	227,100,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount Of Regulatory Costs Not Yet Approved	100,000	1,900,000
Regulatory Liabilities Line Items
Regulatory Liability Current	16,579,000	7,959,000
Regulatory Liabilities Long-Term	16,904,000	15,048,000
Regulatory Liabilities, Total	33,500,000	23,000,000
Western Massachusetts Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	9,300,000	9,500,000
Western Massachusetts Electric Company [Member] | Regulatory Liabilities Offsetting Derivative Assets [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	11,600,000	4,800,000
Western Massachusetts Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	9,300,000	5,600,000
Western Massachusetts Electric Company [Member] | WMECO Provision For Rate Refunds Member
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	1,800,000	2,000,000
Western Massachusetts Electric Company [Member] | Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	0	0
Western Massachusetts Electric Company [Member] | Wholesale Transmission Overcollections [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	1,100,000	0
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	400,000	1,100,000
Cost Of Removal [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	186,700,000	194,800,000
Regulatory Liabilities Offsetting Derivative Assets [Member
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	0	38,100,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	124,400,000	95,100,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	67,500,000	62,100,000
Pension Liability Yankee Gas Acquisition [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	11,300,000	12,500,000
Overrecovered Spent Nuclear Fuel Costs And Contractual Obligations [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	14,600,000	14,600,000
Wholesale Transmission Overcollections [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	8,700,000	13,700,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities Line Items
Regulatory Liabilities, Total	$ 13,700,000	$ 8,200,000