INCOME TAXES (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	$ 8,800,000	$ (30,900,000)	$ 3,000,000
Current State and Local Tax Expense (Benefit)	(9,400,000)	17,600,000	(26,000,000)
Total Current Income Tax Expense (Benefit)	(600,000)	(13,300,000)	(23,000,000)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	386,200,000	291,300,000	187,700,000
Deferred State and Local Income Tax Expense (Benefit)	45,400,000	800,000	9,100,000
Total Deferred Income Taxes	431,413,000	292,000,000	196,761,000
Investment Tax Credit	(4,100,000)	(3,900,000)	(2,800,000)
Income Tax Expense	426,941,000	274,926,000	170,953,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	1,220,630,000	808,003,000	571,466,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	427,200,000	282,800,000	200,000,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(7,400,000)	(10,800,000)	(14,200,000)
Investment Tax Credit	(4,100,000)	(3,900,000)	(2,800,000)
Income Tax Reconciliation, Tax Credits, Other	(3,700,000)	(3,800,000)	(3,500,000)
Income Tax Reconciliation, State and Local Income Taxes	27,600,000	4,400,000	22,100,000
Income Tax Reconciliation Medicare Subsidy	(8,000,000)	(6,400,000)	(2,200,000)
Tax Asset Valuation Allowance / Reserve Adjustments	(4,300,000)	7,600,000	(33,100,000)
Income Tax Reconciliation, Other Adjustments	(400,000)	5,000,000	4,700,000
Income Tax Expense	426,941,000	274,926,000	170,953,000
Effective Income Tax Rate, Continuing Operations	35.00%	34.00%	29.90%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	435,200,000	811,400,000
Deferred Tax Assets, Derivative Instruments	272,900,000	380,600,000
Deferred Tax Assets, Regulatory Assets and Liabilities	272,700,000	257,900,000
Deferred Tax Assets, Allowance for Doubtful Accounts	65,000,000	64,200,000
Deferred Tax Assets Tax Effect Tax Regulatory Assets	16,200,000	17,200,000
Deferred Tax Assets, Operating Loss Carryforwards	158,000,000	214,600,000
Purchase Accounting Deferred Tax Asset	132,800,000	146,400,000
Deferred Tax Assets, Other	230,600,000	242,400,000
Deferred Tax Assets, Gross	1,583,400,000	2,134,700,000
Deferred Tax Assets, Valuation Allowance	24,300,000	4,200,000
Deferred Tax Assets, Net	1,559,100,000	2,130,500,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	3,806,500,000	3,468,800,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	95,100,000	89,600,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	1,146,700,000	1,561,100,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	248,200,000	217,200,000
Deferred Tax Liabilities Goodwill Regulatory Asset	211,500,000	210,900,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	30,100,000	36,200,000
Deferred Tax Liabilities Regulatory Assets Securitized Contract Termination Costs	300,000	16,600,000
Deferred Tax Liabilities, Other	156,800,000	136,100,000
Deferred Tax Liabilities, Total	5,695,200,000	5,736,500,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	83,100,000	73,500,000	101,200,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	8,200,000	10,300,000	8,000,000
Unrecognized Tax Benefits, Increases Resulting from Prior Period Tax Positions		100,000
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions	(1,100,000)	(800,000)	(35,700,000)
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	(2,200,000)
Unrecognized Tax Benefits, Ending Balance	38,200,000	83,100,000	73,500,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense	(8,600,000)	3,100,000	(2,800,000)
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	1,500,000	10,100,000
Income Tax Uncertainties [Abstract]
Income Tax Examination Description	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. During 2011, NU recorded an after-tax benefit of $29.1 million related to various state tax settlements and certain other adjustments. This benefit was recorded as a reduction to both interest expense and income tax expense (including NU and CL&P tax expense reductions of approximately $22.4 million).
Tax Adjustments, Settlements - Total Impact	(13,600,000)		(29,100,000)
Tax Adjustments, Settlements - Tax Expense Impact	(8,400,000)		(22,400,000)
Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	451,300,000	606,900,000
Tax Credit Carryforward, Amount	8,000,000	3,800,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	12,100,000	74,900,000
Tax Credit Carryforward, Amount	104,700,000	110,200,000
Operating Loss Carryforwards, Valuation Allowance		300,000
The Connecticut Light And Power Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	20,100,000	(47,800,000)	13,900,000
Current State and Local Tax Expense (Benefit)	(6,700,000)	3,100,000	(34,400,000)
Total Current Income Tax Expense (Benefit)	13,400,000	(44,700,000)	(20,500,000)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	114,900,000	141,500,000	106,400,000
Deferred State and Local Income Tax Expense (Benefit)	15,100,000	(500,000)	6,200,000
Total Deferred Income Taxes	130,038,000	140,993,000	112,620,000
Investment Tax Credit	(1,700,000)	(1,900,000)	(2,100,000)
Income Tax Expense	141,663,000	94,437,000	90,033,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	421,075,000	304,162,000	340,197,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	147,400,000	106,500,000	119,100,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(7,000,000)	(9,000,000)	(8,100,000)
Investment Tax Credit	(1,700,000)	(1,900,000)	(2,100,000)
Income Tax Reconciliation, Tax Credits, Other	0	0	(100,000)
Income Tax Reconciliation, State and Local Income Taxes	5,000,000	100,000	4,000,000
Income Tax Reconciliation Medicare Subsidy	0	0	0
Tax Asset Valuation Allowance / Reserve Adjustments	400,000	1,600,000	(22,300,000)
Income Tax Reconciliation, Other Adjustments	(2,400,000)	(2,900,000)	(500,000)
Income Tax Expense	141,663,000	94,437,000	90,033,000
Effective Income Tax Rate, Continuing Operations	33.60%	31.00%	26.50%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	56,000,000	141,200,000
Deferred Tax Assets, Derivative Instruments	272,400,000	375,900,000
Deferred Tax Assets, Regulatory Assets and Liabilities	61,500,000	35,500,000
Deferred Tax Assets, Allowance for Doubtful Accounts	31,200,000	30,400,000
Deferred Tax Assets Tax Effect Tax Regulatory Assets	4,700,000	5,200,000
Deferred Tax Assets, Operating Loss Carryforwards	51,000,000	82,000,000
Deferred Tax Assets, Other	75,300,000	82,800,000
Deferred Tax Assets, Gross	552,100,000	753,000,000
Deferred Tax Assets, Valuation Allowance	23,100,000
Deferred Tax Assets, Net	529,000,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,238,100,000	1,194,700,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	49,300,000	44,400,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	550,400,000	677,700,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	160,100,000	151,800,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	29,000,000	36,200,000
Deferred Tax Liabilities Regulatory Assets Securitized Contract Termination Costs	0	0
Deferred Tax Liabilities, Other	20,600,000	10,100,000
Deferred Tax Liabilities, Total	2,047,500,000	2,114,900,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	49,000,000	46,500,000	80,800,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	2,100,000	2,500,000	1,400,000
Unrecognized Tax Benefits, Increases Resulting from Prior Period Tax Positions		0
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions	(300,000)	0	(35,700,000)
Unrecognized Tax Benefits, Decreases Resulting from Settlements with Taxing Authorities	(39,400,000)
Unrecognized Tax Benefits, Ending Balance	11,400,000	49,000,000	46,500,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense	(4,000,000)	1,300,000	(3,700,000)
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	0	4,000,000
Income Tax Uncertainties [Abstract]
Income Tax Examination Description	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. During 2011, NU recorded an after-tax benefit of $29.1 million related to various state tax settlements and certain other adjustments. This benefit was recorded as a reduction to both interest expense and income tax expense (including NU and CL&P tax expense reductions of approximately $22.4 million).
Tax Adjustments, Settlements - Total Impact	(6,900,000)
Tax Adjustments, Settlements - Tax Expense Impact	(4,500,000)		(22,400,000)
The Connecticut Light And Power Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	145,800,000	234,300,000
Tax Credit Carryforward, Amount	0	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	86,800,000	75,200,000
NSTAR Electric Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	95,800,000	93,500,000	64,900,000
Current State and Local Tax Expense (Benefit)	29,600,000	27,600,000	30,200,000
Total Current Income Tax Expense (Benefit)	125,400,000	121,100,000	95,100,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	49,800,000	11,400,000	74,800,000
Deferred State and Local Income Tax Expense (Benefit)	(1,000,000)	(7,100,000)	(2,800,000)
Total Deferred Income Taxes	48,808,000	4,264,000	72,006,000
Investment Tax Credit	(1,300,000)	(1,400,000)	(1,400,000)
Income Tax Expense	172,866,000	123,966,000	165,686,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	441,412,000	314,208,000	418,180,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	154,500,000	110,000,000	146,400,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	100,000	0	0
Investment Tax Credit	(1,300,000)	(1,400,000)	(1,400,000)
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	18,600,000	13,400,000	17,900,000
Income Tax Reconciliation Medicare Subsidy	0	0	0
Tax Asset Valuation Allowance / Reserve Adjustments	0	0	0
Income Tax Reconciliation, Other Adjustments	1,000,000	2,000,000	2,800,000
Income Tax Expense	172,866,000	123,966,000	165,686,000
Effective Income Tax Rate, Continuing Operations	39.20%	39.50%	39.60%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	38,300,000	116,300,000
Deferred Tax Assets, Derivative Instruments	3,300,000	5,800,000
Deferred Tax Assets, Regulatory Assets and Liabilities	114,700,000	123,600,000
Deferred Tax Assets, Allowance for Doubtful Accounts	15,400,000	16,200,000
Deferred Tax Assets Tax Effect Tax Regulatory Assets	5,400,000	6,000,000
Deferred Tax Assets, Operating Loss Carryforwards	0	0
Deferred Tax Assets, Other	31,300,000	26,000,000
Deferred Tax Assets, Gross	208,400,000	293,900,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,179,400,000	1,079,300,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	25,300,000	23,100,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	276,200,000	379,600,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	36,000,000	20,900,000
Deferred Tax Liabilities Goodwill Regulatory Asset	181,600,000	181,000,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	500,000	0
Deferred Tax Liabilities Regulatory Assets Securitized Contract Termination Costs	0	5,500,000
Deferred Tax Liabilities, Other	26,400,000	30,200,000
Deferred Tax Liabilities, Total	1,725,400,000	1,719,600,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense	0	0	2,000,000
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	0	0
NSTAR Electric Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	0	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)	(8,200,000)	(900,000)	(25,800,000)
Current State and Local Tax Expense (Benefit)	3,600,000	3,400,000	100,000
Total Current Income Tax Expense (Benefit)	(4,600,000)	2,500,000	(25,700,000)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	64,500,000	46,500,000	67,700,000
Deferred State and Local Income Tax Expense (Benefit)	11,200,000	12,000,000	7,900,000
Total Deferred Income Taxes	75,693,000	58,552,000	75,628,000
Investment Tax Credit	0	0	0
Income Tax Expense	71,101,000	60,993,000	49,945,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	182,498,000	157,875,000	150,212,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	63,900,000	55,300,000	52,600,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	600,000	(300,000)	(4,400,000)
Investment Tax Credit	0	0	0
Income Tax Reconciliation, Tax Credits, Other	(3,700,000)	(3,800,000)	(3,400,000)
Income Tax Reconciliation, State and Local Income Taxes	9,600,000	10,000,000	5,200,000
Income Tax Reconciliation Medicare Subsidy	0	0	0
Tax Asset Valuation Allowance / Reserve Adjustments	0	0	0
Income Tax Reconciliation, Other Adjustments	700,000	(200,000)	(100,000)
Income Tax Expense	71,101,000	60,993,000	49,945,000
Effective Income Tax Rate, Continuing Operations	39.00%	38.60%	33.20%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	15,500,000	64,800,000
Deferred Tax Assets, Derivative Instruments	0	0
Deferred Tax Assets, Regulatory Assets and Liabilities	40,900,000	43,900,000
Deferred Tax Assets, Allowance for Doubtful Accounts	3,100,000	2,900,000
Deferred Tax Assets Tax Effect Tax Regulatory Assets	2,100,000	1,700,000
Deferred Tax Assets, Operating Loss Carryforwards	56,600,000	71,400,000
Deferred Tax Assets, Other	40,300,000	33,700,000
Deferred Tax Assets, Gross	158,500,000	218,400,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	526,600,000	476,500,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	7,100,000	6,800,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	109,300,000	149,300,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	16,300,000	15,800,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	0
Deferred Tax Liabilities Regulatory Assets Securitized Contract Termination Costs	0	7,900,000
Deferred Tax Liabilities, Other	28,000,000	14,100,000
Deferred Tax Liabilities, Total	687,300,000	670,400,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense	0	0	(600,000)
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	0	0
Public Service Company Of New Hampshire [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	161,800,000	204,000,000
Tax Credit Carryforward, Amount	7,600,000	3,800,000
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)	(53,400,000)	(24,700,000)	100,000
Current State and Local Tax Expense (Benefit)	4,200,000	3,400,000	300,000
Total Current Income Tax Expense (Benefit)	(49,200,000)	(21,300,000)	400,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	84,700,000	51,200,000	22,100,000
Deferred State and Local Income Tax Expense (Benefit)	2,300,000	2,700,000	1,000,000
Total Deferred Income Taxes	87,028,000	53,942,000	23,056,000
Investment Tax Credit	(400,000)	(500,000)	(300,000)
Income Tax Expense	37,368,000	32,140,000	23,186,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	97,806,000	86,643,000	66,240,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	34,200,000	30,300,000	23,200,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	0	200,000	100,000
Investment Tax Credit	(400,000)	(500,000)	(300,000)
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	4,200,000	4,000,000	900,000
Income Tax Reconciliation Medicare Subsidy	0	0	0
Tax Asset Valuation Allowance / Reserve Adjustments	0	0	0
Income Tax Reconciliation Regulatory Decision Non-Plant Flow Through	0	(1,300,000)	0
Income Tax Reconciliation, Other Adjustments	(600,000)	(600,000)	(700,000)
Income Tax Expense	37,368,000	32,140,000	23,186,000
Effective Income Tax Rate, Continuing Operations	38.20%	37.10%	35.00%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	(1,800,000)	16,300,000
Deferred Tax Assets, Derivative Instruments	(2,900,000)	(1,700,000)
Deferred Tax Assets, Regulatory Assets and Liabilities	1,000,000	6,300,000
Deferred Tax Assets, Allowance for Doubtful Accounts	3,300,000	3,200,000
Deferred Tax Assets Tax Effect Tax Regulatory Assets	1,600,000	1,700,000
Deferred Tax Assets, Operating Loss Carryforwards	18,600,000	15,100,000
Deferred Tax Assets, Other	8,300,000	8,000,000
Deferred Tax Assets, Gross	28,100,000	48,900,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	361,100,000	261,300,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	5,900,000	5,100,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	49,300,000	74,500,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	18,200,000	13,900,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	0
Deferred Tax Liabilities Regulatory Assets Securitized Contract Termination Costs	300,000	3,300,000
Deferred Tax Liabilities, Other	3,300,000	2,300,000
Deferred Tax Liabilities, Total	438,100,000	360,400,000
Western Massachusetts Electric Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	53,300,000	43,300,000
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
Northeast Utilities | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	$ 23,700,000