INCOME TAXES (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	$ 4,400,000	$ 8,800,000	$ (30,900,000)
Current State and Local Tax Expense (Benefit)	24,500,000	(9,400,000)	17,600,000
Total Current Income Tax Expense (Benefit)	28,900,000	(600,000)	(13,300,000)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	406,800,000	386,200,000	291,300,000
Deferred State and Local Income Tax Expense (Benefit)	36,500,000	45,400,000	800,000
Total Deferred Income Taxes	443,259,000	431,413,000	292,000,000
Investment Tax Credit	(3,900,000)	(4,100,000)	(3,900,000)
Income Tax Expense	468,297,000	426,941,000	274,926,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	1,295,362,000	1,220,630,000	808,003,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	453,400,000	427,200,000	282,800,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(5,600,000)	(7,400,000)	(10,800,000)
Investment Tax Credit	(3,900,000)	(4,100,000)	(3,900,000)
Income Tax Reconciliation, Tax Credits, Other	(3,500,000)	(3,700,000)	(3,800,000)
Income Tax Reconciliation, State and Local Income Taxes	42,500,000	27,600,000	4,400,000
Income Tax Rate Reconciliation Deductions Employee Stock Ownership Plan Dividends	(8,000,000)	(8,000,000)	(6,400,000)
Tax Asset Valuation Allowance / Reserve Adjustments	(2,900,000)	(4,300,000)	7,600,000
Income Tax Reconciliation, Other Adjustments	(3,700,000)	(400,000)	5,000,000
Income Tax Expense	468,297,000	426,941,000	274,926,000
Effective Income Tax Rate, Continuing Operations	36.20%	35.00%	34.00%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	632,200,000	435,200,000
Deferred Tax Assets, Derivative Instruments	199,600,000	272,900,000
Deferred Tax Assets, Regulatory Assets and Liabilities	366,700,000	272,700,000
Deferred Tax Assets, Allowance for Doubtful Accounts	60,500,000	65,000,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	10,000,000	16,200,000
Deferred Tax Assets, Operating Loss Carryforwards	59,100,000	158,000,000
Purchase Accounting Deferred Tax Asset	126,200,000	132,800,000
Deferred Tax Assets, Other	198,700,000	230,600,000
Deferred Tax Assets, Gross	1,653,000,000	1,583,400,000
Deferred Tax Assets, Valuation Allowance	5,100,000	24,300,000
Deferred Tax Assets, Net	1,647,900,000	1,559,100,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	4,215,900,000	3,806,500,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	109,600,000	95,100,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	1,277,900,000	1,146,700,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	240,200,000	248,200,000
Deferred Tax Liabilities Goodwill Regulatory Asset	203,200,000	211,500,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	32,600,000	30,100,000
Deferred Tax Liabilities, Other	196,300,000	157,100,000
Deferred Tax Liabilities, Total	6,275,700,000	5,695,200,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	38,200,000	83,100,000	73,500,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	9,300,000	8,200,000	10,300,000
Unrecognized Tax Benefits, Increases Resulting from Prior Period Tax Positions	300,000		100,000
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions		(1,100,000)	(800,000)
Unrecognized Tax Benefits, Decreases Resulting from Settlements with Taxing Authorities		(49,800,000)
Unrecognized Tax Benefits, Reductions Resulting from Lapse of Applicable Statute of Limitations	(1,600,000)	(2,200,000)
Unrecognized Tax Benefits, Ending Balance	46,200,000	38,200,000	83,100,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense	400,000	(8,600,000)	3,100,000
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	1,900,000	1,500,000
Income Tax Uncertainties [Abstract]
Income Tax Examination Description	Tax Positions: During 2014, NU did not resolve any of its uncertain tax positions.	During 2013, NU received a Final Determination from the Connecticut Department of Revenue Services (DRS) that concluded its audit of NU's Connecticut income tax returns for the years 2005 through 2008. The DRS Determination resulted in total NU and CL&P after-tax benefits of $13.6 million and $6.9 million, respectively, that included a reduction in NU and CL&P pre-tax interest expense of $8.7 million and $4 million, or $5.2 million and $2.4 million after-tax, respectively. Further, the income tax expense impact resulted in a tax benefit to NU and CL&P of $8.4 million and $4.5 million after-tax, respectively.
Tax Adjustments, Settlements - Total Impact		(13,600,000)
Tax Adjustments, Settlements - Tax Expense Impact		(8,400,000)
Tax Years [Abstract]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Other Information	NU estimates that during the next twelve months, differences of a non-timing nature could be resolved, resulting in a zero to $2 million decrease in unrecognized tax benefits by NU. These estimated changes are not expected to have a material impact on the earnings of NU. Other companies' impacts are not expected to be material.
Other Tax Disclosures [Abstract]
Other Information Pertaining To Income Taxes	2014 Federal Legislation: On December 19, 2014, the "Tax Increase Prevention Act of 2014" became law, which extended the accelerated deduction of depreciation to businesses through 2014. This extended stimulus provides NU with cash flow benefits of approximately $200 million (approximately $70 million at CL&P, $50 million at NSTAR Electric, $35 million at PSNH, and $15 million at WMECO) in 2015. 2013 Federal Legislation: On January 2, 2013, the "American Taxpayer Relief Act of 2012" became law, which extended the accelerated deduction of depreciation to businesses through 2013. This extended stimulus provided NU with cash flow benefits of approximately $300 million (approximately $95 million at CL&P, $85 million at NSTAR Electric, $35 million at PSNH, and $50 million at WMECO). On September 13, 2013, the Internal Revenue Service issued final Tangible Property regulations that are meant to simplify, clarify and make more administrable previously issued guidance. NU is in compliance with the new regulations, but continues to evaluate several new potential elections. 2013 Massachusetts: On July 24, 2013, Massachusetts enacted a law that changed the income tax rate applicable to utility companies effective January 1, 2014, from 6.5 percent to 8 percent. The tax law change required NU to remeasure its accumulated deferred income taxes and resulted in NU increasing its deferred tax liability with an offsetting regulatory asset of approximately $61 million at its utility companies ($46.3 million at NSTAR Electric and $9.8 million at WMECO).
Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	168,800,000	451,300,000
Tax Credit Carryforward, Amount	16,300,000	8,000,000
Charitable Contribution	19,400,000	33,700,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	40,600,000	12,100,000
Tax Credit Carryforward, Amount	99,700,000	104,700,000
Charitable Contribution	2,100,000	1,000,000
Tax Credit and Operating Loss Carryforward Valuation Allowance	4,400,000	23,700,000
The Connecticut Light And Power Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	(200,000)	20,100,000	(47,800,000)
Current State and Local Tax Expense (Benefit)	4,300,000	(6,700,000)	3,100,000
Total Current Income Tax Expense (Benefit)	4,100,000	13,400,000	(44,700,000)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	138,000,000	114,900,000	141,500,000
Deferred State and Local Income Tax Expense (Benefit)	(7,100,000)	15,100,000	(500,000)
Total Deferred Income Taxes	130,949,000	130,038,000	140,993,000
Investment Tax Credit	(1,500,000)	(1,700,000)	(1,900,000)
Income Tax Expense	133,451,000	141,663,000	94,437,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	421,205,000	421,075,000	304,162,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	147,400,000	147,400,000	106,500,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(3,600,000)	(7,000,000)	(9,000,000)
Investment Tax Credit	(1,500,000)	(1,700,000)	(1,900,000)
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	4,400,000	5,000,000	100,000
Tax Asset Valuation Allowance / Reserve Adjustments	(6,300,000)	400,000	1,600,000
Income Tax Reconciliation, Other Adjustments	(6,900,000)	(2,400,000)	(2,900,000)
Income Tax Expense	133,451,000	141,663,000	94,437,000
Effective Income Tax Rate, Continuing Operations	31.70%	33.60%	31.00%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	129,000,000	56,000,000
Deferred Tax Assets, Derivative Instruments	193,000,000	272,400,000
Deferred Tax Assets, Regulatory Assets and Liabilities	73,900,000	61,500,000
Deferred Tax Assets, Allowance for Doubtful Accounts	32,300,000	31,200,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	3,100,000	4,700,000
Deferred Tax Assets, Operating Loss Carryforwards	0	51,000,000
Deferred Tax Assets, Other	53,800,000	75,300,000
Deferred Tax Assets, Gross	485,100,000	552,100,000
Deferred Tax Assets, Valuation Allowance	4,000,000	23,100,000
Deferred Tax Assets, Net	481,100,000	529,000,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,378,600,000	1,238,100,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	58,100,000	49,300,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	502,300,000	550,400,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	166,900,000	160,100,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	32,600,000	29,000,000
Deferred Tax Liabilities, Other	19,400,000	20,600,000
Deferred Tax Liabilities, Total	2,157,900,000	2,047,500,000
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Tax Credit Carryforward Description	In 2014, the Company recorded a reduction to its state credit carryforwards of $11 million (CL&P $10.1 million), net of tax, as a result of an update to reflect the amounts expired. Further, the Company decreased its valuation allowance reserve for state credits by $19.2 million at CL&P, net of tax, to reflect an update for expired state credits and latest estimate of usage.
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	11,400,000	49,000,000	46,500,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	2,700,000	2,100,000	2,500,000
Unrecognized Tax Benefits, Increases Resulting from Prior Period Tax Positions	200,000		0
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions		(300,000)	0
Unrecognized Tax Benefits, Decreases Resulting from Settlements with Taxing Authorities		(39,400,000)
Unrecognized Tax Benefits, Ending Balance	14,300,000	11,400,000	49,000,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense	0	(4,000,000)	1,300,000
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	0	0
Income Tax Uncertainties [Abstract]
Income Tax Examination Description		During 2013, NU received a Final Determination from the Connecticut Department of Revenue Services (DRS) that concluded its audit of NU's Connecticut income tax returns for the years 2005 through 2008. The DRS Determination resulted in total NU and CL&P after-tax benefits of $13.6 million and $6.9 million, respectively, that included a reduction in NU and CL&P pre-tax interest expense of $8.7 million and $4 million, or $5.2 million and $2.4 million after-tax, respectively. Further, the income tax expense impact resulted in a tax benefit to NU and CL&P of $8.4 million and $4.5 million after-tax, respectively.
Tax Adjustments, Settlements - Total Impact		(6,900,000)
Tax Adjustments, Settlements - Tax Expense Impact		(4,500,000)
The Connecticut Light And Power Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	145,800,000
Tax Credit Carryforward, Amount	100,000	0
The Connecticut Light And Power Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	71,000,000	86,800,000
NSTAR Electric Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	75,000,000	95,800,000	93,500,000
Current State and Local Tax Expense (Benefit)	20,200,000	29,600,000	27,600,000
Total Current Income Tax Expense (Benefit)	95,200,000	125,400,000	121,100,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	88,000,000	49,800,000	11,400,000
Deferred State and Local Income Tax Expense (Benefit)	20,100,000	(1,000,000)	(7,100,000)
Total Deferred Income Taxes	108,133,000	48,808,000	4,264,000
Investment Tax Credit	(1,300,000)	(1,300,000)	(1,400,000)
Income Tax Expense	201,981,000	172,866,000	123,966,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	505,069,000	441,412,000	314,208,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	176,800,000	154,500,000	110,000,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(1,300,000)	100,000	0
Investment Tax Credit	(1,300,000)	(1,300,000)	(1,400,000)
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	26,200,000	18,600,000	13,400,000
Income Tax Reconciliation, Other Adjustments	1,600,000	1,000,000	2,000,000
Income Tax Expense	201,981,000	172,866,000	123,966,000
Effective Income Tax Rate, Continuing Operations	40.00%	39.20%	39.50%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	39,900,000	38,300,000
Deferred Tax Assets, Derivative Instruments	1,800,000	3,300,000
Deferred Tax Assets, Regulatory Assets and Liabilities	181,300,000	114,700,000
Deferred Tax Assets, Allowance for Doubtful Accounts	13,800,000	15,400,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	1,800,000	5,400,000
Deferred Tax Assets, Operating Loss Carryforwards	0	0
Deferred Tax Assets, Other	19,900,000	31,300,000
Deferred Tax Assets, Gross	258,500,000	208,400,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,296,900,000	1,179,400,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	25,000,000	25,300,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	276,000,000	276,200,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	35,500,000	36,000,000
Deferred Tax Liabilities Goodwill Regulatory Asset	174,400,000	181,600,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	500,000
Deferred Tax Liabilities, Other	33,500,000	26,400,000
Deferred Tax Liabilities, Total	1,841,300,000	1,725,400,000
NSTAR Electric Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	200,000	0
NSTAR Electric Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	0	0
Public Service Company Of New Hampshire [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	(22,600,000)	(8,200,000)	(900,000)
Current State and Local Tax Expense (Benefit)	(100,000)	3,600,000	3,400,000
Total Current Income Tax Expense (Benefit)	(22,700,000)	(4,600,000)	2,500,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	79,600,000	64,500,000	46,500,000
Deferred State and Local Income Tax Expense (Benefit)	15,200,000	11,200,000	12,000,000
Total Deferred Income Taxes	94,813,000	75,693,000	58,552,000
Investment Tax Credit	0	0	0
Income Tax Expense	72,135,000	71,101,000	60,993,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	186,079,000	182,498,000	157,875,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	65,100,000	63,900,000	55,300,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	300,000	600,000	(300,000)
Investment Tax Credit	0	0	0
Income Tax Reconciliation, Tax Credits, Other	(3,500,000)	(3,700,000)	(3,800,000)
Income Tax Reconciliation, State and Local Income Taxes	9,800,000	9,600,000	10,000,000
Income Tax Reconciliation, Other Adjustments	400,000	700,000	(200,000)
Income Tax Expense	72,135,000	71,101,000	60,993,000
Effective Income Tax Rate, Continuing Operations	38.70%	39.00%	38.60%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	46,800,000	15,500,000
Deferred Tax Assets, Derivative Instruments	0	0
Deferred Tax Assets, Regulatory Assets and Liabilities	46,500,000	40,900,000
Deferred Tax Assets, Allowance for Doubtful Accounts	3,200,000	3,100,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	2,100,000	2,100,000
Deferred Tax Assets, Operating Loss Carryforwards	32,100,000	56,600,000
Deferred Tax Assets, Other	48,900,000	40,300,000
Deferred Tax Assets, Gross	179,600,000	158,500,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	596,600,000	526,600,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	7,400,000	7,100,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	147,600,000	109,300,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	15,900,000	16,300,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	0
Deferred Tax Liabilities, Other	35,600,000	28,000,000
Deferred Tax Liabilities, Total	803,100,000	687,300,000
Public Service Company Of New Hampshire [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	91,800,000	161,800,000
Tax Credit Carryforward, Amount	11,100,000	7,600,000
Public Service Company Of New Hampshire [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	0	0
Western Massachusetts Electric Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	1,900,000	(53,400,000)	(24,700,000)
Current State and Local Tax Expense (Benefit)	1,800,000	4,200,000	3,400,000
Total Current Income Tax Expense (Benefit)	3,700,000	(49,200,000)	(21,300,000)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	28,100,000	84,700,000	51,200,000
Deferred State and Local Income Tax Expense (Benefit)	6,000,000	2,300,000	2,700,000
Total Deferred Income Taxes	34,108,000	87,028,000	53,942,000
Investment Tax Credit	(500,000)	(400,000)	(500,000)
Income Tax Expense	37,268,000	37,368,000	32,140,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	95,087,000	97,806,000	86,643,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	33,300,000	34,200,000	30,300,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(200,000)	0	200,000
Investment Tax Credit	(500,000)	(400,000)	(500,000)
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	5,000,000	4,200,000	4,000,000
Income Tax Reconciliation, Other Adjustments	(300,000)	(600,000)	(1,900,000)
Income Tax Expense	37,268,000	37,368,000	32,140,000
Effective Income Tax Rate, Continuing Operations	39.20%	38.20%	37.10%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	9,200,000	(1,800,000)
Deferred Tax Assets, Derivative Instruments	0	(2,900,000)
Deferred Tax Assets, Regulatory Assets and Liabilities	11,400,000	1,000,000
Deferred Tax Assets, Allowance for Doubtful Accounts	3,800,000	3,300,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	2,500,000	1,600,000
Deferred Tax Assets, Operating Loss Carryforwards	4,500,000	18,600,000
Deferred Tax Assets, Other	4,900,000	8,300,000
Deferred Tax Assets, Gross	36,300,000	28,100,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	385,800,000	361,100,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	12,800,000	5,900,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	60,400,000	49,300,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	9,300,000	18,200,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	0
Deferred Tax Liabilities, Other	2,800,000	3,600,000
Deferred Tax Liabilities, Total	471,100,000	438,100,000
Western Massachusetts Electric Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	12,700,000	53,300,000
Tax Credit Carryforward, Amount	0	0
Western Massachusetts Electric Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	$ 0	$ 0