REGULATORY ACCOUNTING (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 4,728,800,000	$ 4,726,600,000
Regulatory Assets Current	747,349,000	672,493,000
Regulatory Assets Long Term	3,981,507,000	4,054,086,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	49,300,000	60,500,000
Public Utilities Disclosure Of Regulatory Matters Pending	2015 Regulatory Developments: As a result of the March 3, 2015 FERC order in the pending ROE complaint proceedings described in Note 8C, "Commitments and Contingencies – FERC ROE Complaints," in the first quarter of 2015, Eversource recognized a pre-tax charge to earnings (excluding interest) of $20 million, of which $12.5 million was recorded at CL&P, $2.4 million at NSTAR Electric, $1 million at PSNH, and $4.1 million at WMECO. The pre-tax charge was recorded as a regulatory liability and as a reduction of Operating Revenues. On March 2, 2015, the DPU approved the comprehensive settlement agreement between NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General (the "Settlement") as filed with the DPU on December 31, 2014. The Settlement resolved the outstanding NSTAR Electric CPSL program filings for 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the recovery of LBR related to NSTAR Electric's energy efficiency programs for 2008 through 2011 (11 dockets in total). As a result, NSTAR Electric and NSTAR Gas will refund a combined $44.7 million to customers. The refund was recorded as a regulatory liability as of March 31, 2015 and NSTAR Electric recognized a $21.7 million pre-tax benefit in the first quarter of 2015. For further information, see Note 8D, "Commitments and Contingencies – 2014 Comprehensive Settlement Agreement." On January 7, 2015, the DPU issued an order concluding that NSTAR Electric had appropriately accounted for the removal of supply-related bad debt costs from base distribution rates effective January 1, 2006. The DPU ordered NSTAR Electric and the Massachusetts Attorney General to collaborate on the reconciliation of energy-related bad debt costs through 2014. During the second quarter of 2015, NSTAR Electric expects to file with the DPU to recover from customers approximately $43 million of supply-related bad debt costs. In the first quarter of 2015, as a result of the DPU order, NSTAR Electric increased its regulatory assets and reduced Operations and Maintenance expense by $24.2 million, resulting in an increase in after-tax earnings of $14.5 million. For further information, see Note 8E, "Commitments and Contingencies – Basic Service Bad Debt Adder."
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	726,100,000	750,100,000
Regulatory Liability Current	201,180,000	235,022,000
Regulatory Liabilities Long-Term	524,940,000	515,144,000
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	452,800,000	439,900,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	183,300,000	192,300,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	67,100,000	67,100,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	22,900,000	50,800,000
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,663,800,000	1,695,800,000
Regulatory Assets Current	209,628,000	220,344,000
Regulatory Assets Long Term	1,454,150,000	1,475,508,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	1,600,000	1,300,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	166,200,000	206,000,000
Regulatory Liability Current	84,127,000	124,722,000
Regulatory Liabilities Long-Term	82,110,000	81,298,000
The Connecticut Light And Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	23,100,000	19,700,000
The Connecticut Light And Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	77,500,000	122,600,000
The Connecticut Light And Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	53,300,000	53,600,000
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	12,300,000	10,100,000
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,488,200,000	1,377,800,000
Regulatory Assets Current	309,547,000	198,710,000
Regulatory Assets Long Term	1,178,738,000	1,179,100,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	18,300,000	33,200,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	292,700,000	312,300,000
Regulatory Liability Current	24,605,000	49,611,000
Regulatory Liabilities Long-Term	268,122,000	262,738,000
NSTAR Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	263,400,000	258,300,000
NSTAR Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	22,500,000	20,700,000
NSTAR Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,700,000	4,400,000
NSTAR Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,100,000	28,900,000
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	387,200,000	404,800,000
Regulatory Assets Current	99,994,000	111,705,000
Regulatory Assets Long Term	287,203,000	293,115,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	400,000	900,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	68,000,000	67,400,000
Regulatory Liability Current	16,102,000	16,044,000
Regulatory Liabilities Long-Term	51,897,000	51,372,000
Public Service Company Of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	51,300,000	50,300,000
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	13,900,000	14,200,000
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,800,000	2,900,000
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	204,600,000	198,200,000
Regulatory Assets Current	66,706,000	51,923,000
Regulatory Assets Long Term	137,894,000	146,307,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	11,800,000	11,000,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	35,000,000	33,300,000
Regulatory Liability Current	22,289,000	22,486,000
Regulatory Liabilities Long-Term	12,673,000	10,835,000
Western Massachusetts Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,800,000	1,100,000
Western Massachusetts Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	22,200,000	22,300,000
Western Massachusetts Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	9,100,000	9,100,000
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	900,000	800,000
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,976,600,000	2,016,000,000
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	436,700,000	445,400,000
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	505,600,000	515,900,000
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	171,200,000	174,300,000
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	83,300,000	85,000,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	410,200,000	425,500,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	403,300,000	410,900,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	3,500,000	4,500,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Regulatory Assets Offsetting Derivative Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Deferred Income Tax Charges Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	632,100,000	635,300,000
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	438,700,000	437,700,000
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	83,700,000	83,700,000
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	36,800,000	38,000,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,200,000	35,500,000
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	504,800,000	502,800,000
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	308,500,000	319,600,000
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	119,700,000	103,700,000
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	46,900,000	47,700,000
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	29,700,000	31,800,000
Goodwill Regulatory Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	500,200,000	505,400,000
Goodwill Regulatory Asset [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	429,500,000	433,900,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	434,500,000	350,500,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	10,100,000	16,100,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	261,200,000	141,400,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	93,400,000	103,500,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	47,600,000	33,000,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	119,000,000	123,800,000
Power Contracts Buy Out Agreements [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total
Power Contracts Buy Out Agreements [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total
Power Contracts Buy Out Agreements [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	151,400,000	167,300,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	66,500,000	66,100,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	85,000,000	94,700,000
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	38,900,000	41,300,000
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$ 12,800,000	$ 12,900,000