LONG-TERM DEBT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	$ 228,883	$ 245,583
Long-Term Debt	8,805,574	8,568,429
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	200,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	745,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	960,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	800,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	295,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	5,736,600
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	8,736,600
The Connecticut Light And Power Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	0	162,000
Long-Term Debt	2,763,682	2,664,243
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	0
Long-term Debt, Maturities, Repayments of Principal in Year Two	250,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	300,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	250,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	0
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,990,300
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	$ 2,790,300
Long-term Debt, Description	On May 20, 2015 and December 1, 2015, CL&P issued $300 million and $50 million, respectively, of 4.15 percent 2015 Series A First and Refunding Mortgage Bonds due to mature in 2045. : On April 1, 2015, CL&P repaid at maturity the $100 million 5.00 percent 2005 Series A First and Refunding Mortgage Bonds and also redeemed the $62 million 1996A Series 1.55 percent PCRBs that were subject to mandatory tender using short-term borrowings.
Repayment of spent nuclear fuel	$ 244,600
Interest included in payment to settle spent nuclear fuel obligation	178,000
The Connecticut Light And Power Company [Member] | Series D 1994 Due 2024 7.875 % [Member]
Debt Instrument [Line Items]
Long-Term Debt	139,800	139,800
The Connecticut Light And Power Company [Member] | Series B 2004 Due 2034 5.750% [Member]
Debt Instrument [Line Items]
Long-Term Debt	130,000	130,000
The Connecticut Light And Power Company [Member] | Series A 2005 Due 2015 5.000% [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	100,000
The Connecticut Light And Power Company [Member] | Series B 2005 Due 2035 5.625% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000	100,000
The Connecticut Light And Power Company [Member] | Series A 2006 Due 2036 6.350% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000	250,000
The Connecticut Light And Power Company [Member] | Series A 2007 Due 2017 5.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000	150,000
The Connecticut Light And Power Company [Member] | Series B 2007 Due 2037 5.750% [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000	150,000
The Connecticut Light And Power Company [Member] | Series D 2007 Due 2037 6.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000	100,000
The Connecticut Light And Power Company [Member] | Series A 2008 Due 2018 5.650% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000	300,000
The Connecticut Light And Power Company [Member] | Series A 2009 Due 2019 5.500% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000	250,000
The Connecticut Light And Power Company [Member] | One-Year Fixed Rate Tax Exempt Due 2031 [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	62,000
The Connecticut Light And Power Company [Member] | Fees And Interest Due For Spent Nuclear Fuel Disposal Costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	244,500
The Connecticut Light And Power Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	2,669,800	2,419,800
The Connecticut Light And Power Company [Member] | Total Pollution Control Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	120,500	182,500
The Connecticut Light And Power Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	(10,700)	(4,800)
The Connecticut Light And Power Company [Member] | Serires C 2007 5.750% Due 2017 [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000	100,000
The Connecticut Light And Power Company [Member] | Fixed Rate 4.375% due 2028 [Member]
Debt Instrument [Line Items]
Long-Term Debt	120,500	120,500
The Connecticut Light And Power Company [Member] | Series A Due 2023 2.5 percent [Member]
Debt Instrument [Line Items]
Long-Term Debt	400,000	400,000
The Connecticut Light And Power Company [Member] | Series A 2014 Due 2044 4.30% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000	250,000
The Connecticut Light And Power Company [Member] | Series O First Mortgage Bonds due 2045 4.35% [Member]
Debt Instrument [Line Items]
Long-Term Debt	350,000
The Connecticut Light And Power Company [Member] | Unamortized Debt issuance costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	(15,900)	(15,800)
NSTAR Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	200,000	4,700
Long-Term Debt	1,829,766	1,781,541
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	200,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	400,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	0
Long-term Debt, Maturities, Repayments of Principal in Year Four	0
Long-term Debt, Maturities, Repayments of Principal in Year Five	0
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,450,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	$ 2,050,000
Long-term Debt, Description	. On November 18, 2015, NSTAR Electric issued $250 million of 3.25 percent debentures, due to mature in 2025.
NSTAR Electric Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	$ (8,500)	(7,300)
NSTAR Electric Company [Member] | Total Other [Member]
Debt Instrument [Line Items]
Long-Term Debt	2,050,000	1,800,000
NSTAR Electric Company [Member] | Due 2022 2.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	400,000	400,000
NSTAR Electric Company [Member] | Due 2017 5.625% [Member]
Debt Instrument [Line Items]
Long-Term Debt	400,000	400,000
NSTAR Electric Company [Member] | Due 2036 5.75% [Member]
Debt Instrument [Line Items]
Long-Term Debt	200,000	200,000
NSTAR Electric Company [Member] | Due 2040 5.5% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000	300,000
NSTAR Electric Company [Member] | TaxExempt Sewage Facility Revenue Bonds Due 2015 [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	4,700
NSTAR Electric Company [Member] | Variable Due 2016 [Member]
Debt Instrument [Line Items]
Long-Term Debt	200,000	200,000
NSTAR Electric Company [Member] | Debentures 2014 Due 2044 4.40% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000	300,000
NSTAR Electric Company [Member] | Debentures Due 2025 3.25% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000
NSTAR Electric Company [Member] | Unamortized Debt issuance costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	(11,700)	(11,200)
Public Service Company Of New Hampshire [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,071,017	1,070,021
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	0
Long-term Debt, Maturities, Repayments of Principal in Year Two	70,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	110,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	150,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	0
Long-term Debt, Maturities, Repayments of Principal after Year Five	746,300
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	1,076,300
Public Service Company Of New Hampshire [Member] | Series M 2005 Due 2035 5.60% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000	50,000
Public Service Company Of New Hampshire [Member] | Series N 2007 Due 2017 6.15% [Member]
Debt Instrument [Line Items]
Long-Term Debt	70,000	70,000
Public Service Company Of New Hampshire [Member] | Series O 2008 Due 2018 6.00% [Member]
Debt Instrument [Line Items]
Long-Term Debt	110,000	110,000
Public Service Company Of New Hampshire [Member] | Series P 2009 Due 2019 4.50% [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000	150,000
Public Service Company Of New Hampshire [Member] | Adjustablerateseriesadue 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	89,300	89,300
Public Service Company Of New Hampshire [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	987,000	987,000
Public Service Company Of New Hampshire [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	100	0
Public Service Company Of New Hampshire [Member] | Series Q 4.05% 2011 Due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	122,000	122,000
Public Service Company Of New Hampshire [Member] | Series R 2011 3.30% due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	160,000	160,000
Public Service Company Of New Hampshire [Member] | Series2013due2023member [Member]
Debt Instrument [Line Items]
Long-Term Debt	325,000	325,000
Public Service Company Of New Hampshire [Member] | Unamortized Debt issuance costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	(5,400)	(6,300)
Western Massachusetts Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	0	50,000
Long-Term Debt	517,329	575,184
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	0
Long-term Debt, Maturities, Repayments of Principal in Year Two	0
Long-term Debt, Maturities, Repayments of Principal in Year Three	0
Long-term Debt, Maturities, Repayments of Principal in Year Four	0
Long-term Debt, Maturities, Repayments of Principal in Year Five	95,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	420,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	$ 515,000
Long-term Debt, Description	On August 3, 2015, WMECO repaid at maturity the $50 million 5.24 percent Series C Senior Notes, using short-term borrowings.
Repayment of spent nuclear fuel	$ 57,400
Interest included in payment to settle spent nuclear fuel obligation	41,800
Western Massachusetts Electric Company [Member] | Senior Notes Series B 5.90% Due 2034 [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000	50,000
Western Massachusetts Electric Company [Member] | Senior Notes Series C 5.24% Due 2015 [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	50,000
Western Massachusetts Electric Company [Member] | Senior Notes Series D 6.70% Due 2037 [Member]
Debt Instrument [Line Items]
Long-Term Debt	40,000	40,000
Western Massachusetts Electric Company [Member] | Senior Notes Series E 5.10% Due 2020 [Member]
Debt Instrument [Line Items]
Long-Term Debt	95,000	95,000
Western Massachusetts Electric Company [Member] | Fees And Interest Due For Spent Nuclear Fuel Disposal Costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	57,400
Western Massachusetts Electric Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	5,200	6,100
Western Massachusetts Electric Company [Member] | Senior Notes Series F 3.50% due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000	250,000
Western Massachusetts Electric Company [Member] | Total Pollution Control Notes and Other Notes [Member]
Debt Instrument [Line Items]
Long-Term Debt	515,000	565,000
Western Massachusetts Electric Company [Member] | Series g 2013 due 2023 member [Member]
Debt Instrument [Line Items]
Long-Term Debt	80,000	80,000
Western Massachusetts Electric Company [Member] | Unamortized Debt issuance costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	(2,900)	(3,300)
Yankee Gas Services Company [Member]
Debt Instrument [Line Items]
Long-Term Debt	443,700	369,100
Yankee Gas Services Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	445,000	370,000
Yankee Gas Services Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	400	600
Yankee Gas Services Company [Member] | Series B due 2022 8.48% [Member]
Debt Instrument [Line Items]
Long-Term Debt	20,000	20,000
Yankee Gas Services Company [Member] | Series H due 2019 5.26% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000	50,000
Yankee Gas Services Company [Member] | Series I due 2035 5.35% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000	50,000
Yankee Gas Services Company [Member] | Series J due 2018 6.90% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000	100,000
Yankee Gas Services Company [Member] | Series K due 2020 4.87% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000	50,000
Yankee Gas Services Company [Member] | Series L 2014 Due 2044 4.82 % [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000	100,000
Yankee Gas Services Company [Member] | Unamortized Debt issuance costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	(1,700)	(1,500)
Yankee Gas Services Company [Member] | Series M 2015 due 2025 3.35% [Member]
Debt Instrument [Line Items]
Long-Term Debt	75,000
NSTAR Gas Company [Member]
Debt Instrument [Line Items]
Long-Term Debt	309,200	209,400
NSTAR Gas Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	310,000	210,000
NSTAR Gas Company [Member] | Series J Due 202 9.95% [Member]
Debt Instrument [Line Items]
Long-Term Debt	25,000	25,000
NSTAR Gas Company [Member] | Series K Due 2033 7.11% [Member]
Debt Instrument [Line Items]
Long-Term Debt	35,000	35,000
NSTAR Gas Company [Member] | Series M Due 2017 7.04% [Member]
Debt Instrument [Line Items]
Long-Term Debt	25,000	25,000
NSTAR Gas Company [Member] | Series N Due 2020 4.46% [Member]
Debt Instrument [Line Items]
Long-Term Debt	125,000	125,000
NSTAR Gas Company [Member] | Series O First Mortgage Bonds due 2045 4.35% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000
NSTAR Gas Company [Member] | Unamortized Debt issuance costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	(800)	(600)
Subsidiaries [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	(1,300)	(1,200)
Subsidiaries [Member] | Fair Value Adjustment [Member]
Debt Instrument [Line Items]
Long-Term Debt	173,500	202,300
Subsidiaries [Member] | Total Other Long-Term Debt [Member]
Debt Instrument [Line Items]
Long-Term Debt	2,623,800	2,477,500
Subsidiaries [Member] | Total Other [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,550,000	1,546,300
Subsidiaries [Member] | Commerical Paper and Revolver Borrowings [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	446,300
Subsidiaries [Member] | Debentures Due 2019 4.50% [Member]
Debt Instrument [Line Items]
Long-Term Debt	350,000	350,000
Subsidiaries [Member] | Spent Nuclear Fuel Obilgation CY [Member]
Debt Instrument [Line Items]
Long-Term Debt	179,500	179,400
Subsidiaries [Member] | FairValueAdjustmentMemberCurrentPortion [Member]
Debt Instrument [Line Items]
Long-Term Debt	28,900	28,900
Subsidiaries [Member] | Series F Due 2023 2.8% [Member]
Debt Instrument [Line Items]
Long-Term Debt	450,000	450,000
Subsidiaries [Member] | Series E Due 2018 1.45% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000	300,000
Subsidiaries [Member] | Unamortized Debt issuance costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	(1,900)	1,100
Subsidiaries [Member] | Senior Notes Series G 1.6% due 2018 [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000
Subsidiaries [Member] | Senior Notes Series H 3.15% due 2025 [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000
Subsidiaries [Member] | Interest Included in Spent Nuclear Fuel Obligation [Member]
Debt Instrument [Line Items]
Long-Term Debt	$ 130,700	$ 130,600