ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) (USD $)	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	$ (74,009,000)	$ (46,000,000)
Other Comprehensive Income Before Reclassifications, Net of Tax	100,000	200,000
Reclassification From OCI, Net Of Tax	1,500,000	1,500,000
Other Comprehensive Income (Loss), Net of Tax, Total	1,595,000	1,710,000
AOCI Ending Balance	(72,414,000)	(44,300,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(94,843,000)us-gaap_InterestExpense	(89,975,000)us-gaap_InterestExpense
Income Tax Expense	(153,226,000)us-gaap_IncomeTaxExpenseBenefit	(141,545,000)us-gaap_IncomeTaxExpenseBenefit
Net Income Attributable to Controlling Interests	253,254,000	235,957,000
The Connecticut Light And Power Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(931,000)
Other Comprehensive Income (Loss), Net of Tax, Total	115,000	119,000
AOCI Ending Balance	(816,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(36,624,000)	(34,243,000)
Income Tax Expense	(38,147,000)	(45,541,000)
Public Service Company Of New Hampshire [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(7,369,000)
Other Comprehensive Income (Loss), Net of Tax, Total	299,000	304,000
AOCI Ending Balance	(7,070,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(11,272,000)	(11,971,000)
Income Tax Expense	(20,276,000)	(19,700,000)
Western Massachusetts Electric Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(3,176,000)
Other Comprehensive Income (Loss), Net of Tax, Total	86,000	87,000
AOCI Ending Balance	(3,090,000)
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense	(6,823,000)	(5,646,000)
Income Tax Expense	(9,113,000)	(11,558,000)
Qualified Cash Flow Hedging Instruments [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(12,400,000)	(14,400,000)
Reclassification From OCI, Net Of Tax	500,000	500,000
Other Comprehensive Income (Loss), Net of Tax, Total	500,000	500,000
AOCI Ending Balance	(11,900,000)	(13,900,000)
Unrealized Gains Losses on Available for Sale Securities [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	700,000	400,000
Other Comprehensive Income Before Reclassifications, Net of Tax	100,000	200,000
Other Comprehensive Income (Loss), Net of Tax, Total	100,000	200,000
AOCI Ending Balance	800,000	600,000
Pension, SERP and PBOP Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance	(62,300,000)	(32,000,000)
Other Comprehensive Income Before Reclassifications, Net of Tax	0
Reclassification From OCI, Net Of Tax	1,000,000	1,000,000
Other Comprehensive Income (Loss), Net of Tax, Total	1,000,000	1,000,000
AOCI Ending Balance	$ (61,300,000)	$ (31,000,000)

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the cost of borrowed funds accounted for as interest expense.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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