INCOME TAXES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	$ 6,200	$ 4,400	$ 8,800
Current State and Local Tax Expense (Benefit)	45,700	24,500	(9,400)
Total Current Income Tax Expense (Benefit)	51,900	28,900	(600)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	436,100	406,800	386,200
Deferred State and Local Income Tax Expense (Benefit)	55,600	36,500	45,400
Total Deferred Income Taxes	491,736	443,259	431,413
Investment Tax Credit	(3,600)	(3,900)	(4,100)
Income Tax Expense	539,967	468,297	426,941
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	1,425,971	1,295,362	1,220,630
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	499,100	453,400	427,200
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(4,600)	(5,600)	(7,400)
Investment Tax Credit	(3,600)	(3,900)	(4,100)
Income Tax Reconciliation, Tax Credits, Other	(3,800)	(3,500)	(3,700)
Income Tax Reconciliation, State and Local Income Taxes	61,100	42,500	27,600
Income Tax Rate Reconciliation Deductions Employee Stock Ownership Plan Dividends	(8,100)	(8,000)	(8,000)
Tax Asset Valuation Allowance / Reserve Adjustments	4,700	(2,900)	(4,300)
Income Tax Reconciliation, Other Adjustments	(4,800)	(3,700)	(400)
Income Tax Expense	$ 539,967	$ 468,297	$ 426,941
Effective Income Tax Rate, Continuing Operations	37.90%	36.20%	35.00%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	$ 637,500	$ 632,200
Deferred Tax Assets, Derivative Instruments	172,700	199,600
Deferred Tax Assets, Regulatory Assets and Liabilities	243,500	366,700
Deferred Tax Assets, Allowance for Doubtful Accounts	60,500	60,500
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	9,700	10,000
Deferred Tax Assets, Operating Loss Carryforwards	5,400	59,100
Purchase Accounting Deferred Tax Asset	119,300	126,200
Deferred Tax Assets, Other	197,100	198,700
Deferred Tax Assets, Gross	1,445,700	1,653,000
Deferred Tax Assets, Valuation Allowance	3,700	5,100
Deferred Tax Assets, Net	1,442,000	1,647,900
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	4,602,600	4,215,900
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	76,700	109,600
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	1,289,100	1,277,900
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	249,300	240,200
Deferred Tax Liabilities Goodwill Regulatory Asset	194,900	203,200
Deferred Tax Liabilities Regulatory Assets Derivative Assets	17,700	32,600
Deferred Tax Liabilities, Other	159,400	196,300
Deferred Tax Liabilities, Total	$ 6,589,700	6,275,700
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Tax Credit Carryforward Description	In 2015, the Company decreased its valuation allowance reserve for state credits and state loss carryforwards by $1.3 million (CL&P $0.9 million), net of tax, to reflect an update for expired state tax credits and loss carryforwards. 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	$ 46,200	38,200	$ 83,100
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	9,900	9,300	8,200
Unrecognized Tax Benefits, Increases Resulting from Prior Period Tax Positions	100	300
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions			(1,100)
Unrecognized Tax Benefits, Decreases Resulting from Settlements with Taxing Authorities			(49,800)
Unrecognized Tax Benefits, Reductions Resulting from Lapse of Applicable Statute of Limitations	(8,200)	(1,600)	(2,200)
Unrecognized Tax Benefits, Ending Balance	48,000	46,200	38,200
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense	100	400	(8,600)
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	$ 2,000	$ 1,900
Income Tax Uncertainties [Abstract]
Income Tax Examination Description	Tax Positions: During 2015 and 2014, Eversource did not resolve any of its uncertain tax positions.
Tax Years [Abstract]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Other Information	Eversource estimates that during the next twelve months, differences of a non-timing nature could be resolved, resulting in a zero to $2.3 million decrease in unrecognized tax benefits by Eversource. These estimated changes are not expected to have a material impact on the earnings of Eversource. Other companies' impacts are not expected to be material.
Other Tax Disclosures [Abstract]
Other Information Pertaining To Income Taxes	2015 Federal Legislation: On December 18, 2015, the "Protecting Americans from Tax Hikes" Act became law, which extended the accelerated deduction of depreciation to businesses from 2015 through 2019. This extended stimulus provides Eversource with cash flow benefits in 2016 of approximately $275 million (including approximately $105 million for CL&P, $72 million for NSTAR Electric, $46 million for PSNH, and $25 million for WMECO) due to a refund of taxes paid in 2015 and lower expected tax payments in 2016 of approximately $300 million. 2015 Connecticut Legislation: In 2015, the state of Connecticut enacted several changes to its corporate tax laws. Among the changes, commencing as of January 1, 2015, is the reduction in the amount of tax credits that corporations can utilize against its tax liability in a year and a continuation of the corporate income tax surcharge through 2018, which effectively increases the state corporate tax rate to 9 percent for the years 2016 and 2017 and 8.25 percent for 2018. Also, effective January 1, 2016, all Connecticut companies have a mandatory unitary tax filing requirement. Management continues to review the tax law changes and their impact on the effective tax rates of Eversource and CL&P.	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 provided Eversource with cash flow benefits of approximately $250 million (approximately $86 million at CL&P, $64 million at NSTAR Electric, $44 million at PSNH, and $21 million at WMECO) in 2015.
Maximum [Member]
Tax Years [Abstract]
Decrease In Unrecognized Tax Benefits Is Reasonably Possible	$ 2,300
Minimum [Member]
Tax Years [Abstract]
Decrease In Unrecognized Tax Benefits Is Reasonably Possible	0
Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	15,500	$ 168,800
Tax Credit Carryforward, Amount	26,100	16,300
Charitable Contribution	14,900	19,400
State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards		40,600
Tax Credit Carryforward, Amount	101,200	99,700
Charitable Contribution	3,000	2,100
Tax Credit and Operating Loss Carryforward Valuation Allowance	3,100	4,400
The Connecticut Light And Power Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	26,900	(200)	20,100
Current State and Local Tax Expense (Benefit)	15,800	4,300	(6,700)
Total Current Income Tax Expense (Benefit)	42,700	4,100	13,400
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	135,800	138,000	114,900
Deferred State and Local Income Tax Expense (Benefit)	200	(7,100)	15,100
Total Deferred Income Taxes	135,994	130,949	130,038
Investment Tax Credit	(1,300)	(1,500)	(1,700)
Income Tax Expense	177,396	133,451	141,663
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	476,756	421,205	421,075
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	166,900	147,400	147,400
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(1,700)	(3,600)	(7,000)
Investment Tax Credit	(1,300)	(1,500)	(1,700)
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	9,200	4,400	5,000
Tax Asset Valuation Allowance / Reserve Adjustments	1,200	(6,300)	400
Income Tax Reconciliation, Other Adjustments	3,100	(6,900)	(2,400)
Income Tax Expense	$ 177,396	$ 133,451	$ 141,663
Effective Income Tax Rate, Continuing Operations	37.20%	31.70%	33.60%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	$ 126,100	$ 129,000
Deferred Tax Assets, Derivative Instruments	165,700	193,000
Deferred Tax Assets, Regulatory Assets and Liabilities	36,000	73,900
Deferred Tax Assets, Allowance for Doubtful Accounts	30,400	32,300
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	3,100	3,100
Deferred Tax Assets, Operating Loss Carryforwards	0	0
Deferred Tax Assets, Other	55,500	53,800
Deferred Tax Assets, Gross	416,800	485,100
Deferred Tax Assets, Valuation Allowance	3,100	4,000
Deferred Tax Assets, Net	413,700	481,100
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,545,600	1,378,600
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	27,300	58,100
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	456,800	502,300
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	168,700	166,900
Deferred Tax Liabilities Regulatory Assets Derivative Assets	17,700	32,600
Deferred Tax Liabilities, Other	18,500	19,400
Deferred Tax Liabilities, Total	$ 2,234,600	2,157,900
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Tax Credit Carryforward Description	In 2015, the Company decreased its valuation allowance reserve for state credits and state loss carryforwards by $1.3 million (CL&P $0.9 million), net of tax, to reflect an update for expired state tax credits and loss carryforwards. 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	$ 14,300	11,400	$ 49,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	2,600	2,700	2,100
Unrecognized Tax Benefits, Increases Resulting from Prior Period Tax Positions		200
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions			(300)
Unrecognized Tax Benefits, Decreases Resulting from Settlements with Taxing Authorities			(39,400)
Unrecognized Tax Benefits, Reductions Resulting from Lapse of Applicable Statute of Limitations	(3,400)
Unrecognized Tax Benefits, Ending Balance	13,500	14,300	11,400
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense			(4,000)
The Connecticut Light And Power Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	100	100
The Connecticut Light And Power Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards		0
Tax Credit Carryforward, Amount	73,800	71,000
NSTAR Electric Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	36,300	75,000	95,800
Current State and Local Tax Expense (Benefit)	19,800	20,200	29,600
Total Current Income Tax Expense (Benefit)	56,100	95,200	125,400
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	147,500	88,000	49,800
Deferred State and Local Income Tax Expense (Benefit)	25,700	20,100	(1,000)
Total Deferred Income Taxes	173,155	108,133	48,808
Investment Tax Credit	(1,300)	(1,300)	(1,300)
Income Tax Expense	228,044	201,981	172,866
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	572,586	505,069	441,412
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	200,400	176,800	154,500
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(1,400)	(1,300)	100
Investment Tax Credit	(1,300)	(1,300)	(1,300)
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	29,600	26,200	18,600
Income Tax Reconciliation, Other Adjustments	700	1,600	1,000
Income Tax Expense	$ 228,044	$ 201,981	$ 172,866
Effective Income Tax Rate, Continuing Operations	39.80%	40.00%	39.20%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	$ 91,300	$ 39,900
Deferred Tax Assets, Derivative Instruments	600	1,800
Deferred Tax Assets, Regulatory Assets and Liabilities	109,400	181,300
Deferred Tax Assets, Allowance for Doubtful Accounts	8,500	13,800
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	1,500	1,800
Deferred Tax Assets, Operating Loss Carryforwards	0	0
Deferred Tax Assets, Other	3,400	19,900
Deferred Tax Assets, Gross	214,700	258,500
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,387,100	1,296,900
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	22,800	25,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	339,700	276,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	36,000	35,500
Deferred Tax Liabilities Goodwill Regulatory Asset	167,400	174,400
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	0
Deferred Tax Liabilities, Other	22,000	33,500
Deferred Tax Liabilities, Total	1,975,000	1,841,300
NSTAR Electric Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	200	200
NSTAR Electric Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards		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)	(16,700)	(22,600)	$ (8,200)
Current State and Local Tax Expense (Benefit)	6,000	(100)	3,600
Total Current Income Tax Expense (Benefit)	(10,700)	(22,700)	(4,600)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	74,500	79,600	64,500
Deferred State and Local Income Tax Expense (Benefit)	9,300	15,200	11,200
Total Deferred Income Taxes	83,776	94,813	75,693
Investment Tax Credit	0	0	0
Income Tax Expense	73,060	72,135	71,101
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	187,502	186,079	182,498
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	65,600	65,100	63,900
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	500	300	600
Investment Tax Credit	0	0	0
Income Tax Reconciliation, Tax Credits, Other	(3,800)	(3,500)	(3,700)
Income Tax Reconciliation, State and Local Income Taxes	9,900	9,800	9,600
Income Tax Reconciliation, Other Adjustments	900	400	700
Income Tax Expense	$ 73,060	$ 72,135	$ 71,101
Effective Income Tax Rate, Continuing Operations	39.00%	38.70%	39.00%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	$ 37,100	$ 46,800
Deferred Tax Assets, Derivative Instruments	0	0
Deferred Tax Assets, Regulatory Assets and Liabilities	42,100	46,500
Deferred Tax Assets, Allowance for Doubtful Accounts	3,600	3,200
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	2,300	2,100
Deferred Tax Assets, Operating Loss Carryforwards	2,400	32,100
Deferred Tax Assets, Other	61,100	48,900
Deferred Tax Assets, Gross	148,600	179,600
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	655,300	596,600
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	7,300	7,400
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	137,900	147,600
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	15,400	15,900
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	0
Deferred Tax Liabilities, Other	38,600	35,600
Deferred Tax Liabilities, Total	854,500	803,100
Public Service Company Of New Hampshire [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	7,000	91,800
Tax Credit Carryforward, Amount	15,000	11,100
Public Service Company Of New Hampshire [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards		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)	(3,500)	1,900	$ (53,400)
Current State and Local Tax Expense (Benefit)	1,600	1,800	4,200
Total Current Income Tax Expense (Benefit)	(1,900)	3,700	(49,200)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	33,400	28,100	84,700
Deferred State and Local Income Tax Expense (Benefit)	6,000	6,000	2,300
Total Deferred Income Taxes	39,428	34,108	87,028
Investment Tax Credit	(500)	(500)	(400)
Income Tax Expense	36,970	37,268	37,368
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	93,476	95,087	97,806
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	32,700	33,300	34,200
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	(300)	(200)	0
Investment Tax Credit	(500)	(500)	(400)
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	4,900	5,000	4,200
Income Tax Reconciliation, Other Adjustments	200	(300)	(600)
Income Tax Expense	$ 36,970	$ 37,268	$ 37,368
Effective Income Tax Rate, Continuing Operations	39.60%	39.20%	38.20%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	$ 10,000	$ 9,200
Deferred Tax Assets, Derivative Instruments	0	0
Deferred Tax Assets, Regulatory Assets and Liabilities	6,100	11,400
Deferred Tax Assets, Allowance for Doubtful Accounts	4,500	3,800
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	2,400	2,500
Deferred Tax Assets, Operating Loss Carryforwards	400	4,500
Deferred Tax Assets, Other	5,000	4,900
Deferred Tax Assets, Gross	28,400	36,300
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	416,100	385,800
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	10,600	12,800
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	60,500	60,400
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	9,000	9,300
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	0
Deferred Tax Liabilities, Other	2,700	2,800
Deferred Tax Liabilities, Total	498,900	471,100
Western Massachusetts Electric Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	1,000	12,700
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
Tax Credit Carryforward, Amount	$ 0	$ 0