LONG-TERM DEBT (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	$ 245,583,000	$ 533,346,000
Long-Term Debt	8,606,017,000	7,776,833,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	216,700,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	200,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	745,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	810,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	800,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	4,956,600,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	7,728,300,000
Long-term Debt, Description	On September 15, 2014, CL&P repaid at maturity the $150 million of 4.80 percent 2004 Series A First Mortgage Bonds, using short-term borrowings. On April 24, 2014, CL&P issued $250 million of 4.30 percent 2014 Series A First Mortgage Bonds, due to mature in 2044. The proceeds, net of issuance costs, were used to repay short-term borrowings. On February 12, 2015, CL&P notified the trustee that it intends to purchase and cancel the bonds on April 1, 2015, after they have been tendered by the bondholders. On March 7, 2014, NSTAR Electric issued $300 million of 4.40 percent debentures, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $300 million of 4.875 percent debentures that matured on April 15, 2014. As of December 31, 2014 and 2013, the interest rate was 0.4721 percent and 0.478 percent, respectively. On July 15, 2014, PSNH repaid at maturity the $50 million of 5.25 percent Series L First Mortgage Bonds using short-term borrowings. On October 14, 2014, PSNH issued $75 million of first mortgage bonds at a yield of 3.144 percent due to mature in 2023. The first mortgage bonds are part of the same series of PSNH’s existing 3.50 percent Series S First Mortgage Bonds that were initially issued in November 2013. The proceeds, net of issuance costs, were used to repay short-term borrowings. On January 2, 2014, Yankee Gas issued $100 million of 4.82 percent Series L First Mortgage Bonds, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $75 million 4.80 percent Series G First Mortgage Bonds that matured on January 1, 2014 and to pay $25 million in short-term borrowings. As the debt issuance refinanced short-term debt, these amounts were classified as Long-Term Debt on NU's balance sheet as of December 31, 2013. On January 15, 2015, NU parent issued $150 million of 1.60 percent Series G Senior Notes due to mature in 2018 and $300 million of 3.15 percent Series H Notes, due to mature in 2025. The proceeds, net of issuance costs, were used to repay short-term borrowings outstanding under the NU commercial paper program. As the debt issuances refinanced short-term debt, the short-term debt was classified as Long-Term Debt as of December 31, 2014. Amount relates to the purchase price adjustment required to record the NSTAR long-term debt at fair value on the date of the merger. On July 15, 2014, PSNH repaid at maturity the $50 million of 5.25 percent Series L First Mortgage Bonds using short-term borrowings. On October 14, 2014, PSNH issued $75 million of first mortgage bonds at a yield of 3.144 percent due to mature in 2023. The first mortgage bonds are part of the same series of PSNH’s existing 3.50 percent Series S First Mortgage Bonds that were initially issued in November 2013. The proceeds, net of issuance costs, were used to repay short-term borrowings. The DOE is responsible for the selection and development of repositories for, and the disposal of, spent nuclear fuel and high-level radioactive waste. For nuclear fuel used to generate electricity prior to April 7, 1983 (Prior Period Spent Nuclear Fuel) for CL&P and WMECO, an accrual has been recorded for the full liability, and payment must be made by CL&P and WMECO to the DOE prior to the first delivery of spent fuel to the DOE. After the sale of Millstone, CL&P and WMECO remained responsible for their share of the disposal costs associated with the Prior Period Spent Nuclear Fuel. Until such payment to the DOE is made, the outstanding liability will continue to accrue interest at the 3-month Treasury bill yield rate. In addition, as a result of consolidating CYAPC, NU has consolidated $179.4 million in additional spent nuclear fuel obligations, including interest, as of both December 31, 2014 and 2013. The obligation due to the DOE for the disposal of CL&P's and WMECO's Prior Period Spent Nuclear Fuel and CYAPC's spent nuclear fuel obligation include accumulated interest costs of $350.4 million and $350.3 million ($178 million and $177.9 million for CL&P and $41.8 million and $41.7 million for WMECO) as of December 31, 2014 and 2013, respectively.
The Connecticut Light And Power Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	162,000,000	150,000,000
Long-Term Debt	2,679,951,000	2,591,208,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	162,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	0
Long-term Debt, Maturities, Repayments of Principal in Year Three	250,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	300,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	250,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,640,300,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	2,602,300,000
Long-term Debt, Description	On September 15, 2014, CL&P repaid at maturity the $150 million of 4.80 percent 2004 Series A First Mortgage Bonds, using short-term borrowings. On April 24, 2014, CL&P issued $250 million of 4.30 percent 2014 Series A First Mortgage Bonds, due to mature in 2044. The proceeds, net of issuance costs, were used to repay short-term borrowings. On February 12, 2015, CL&P notified the trustee that it intends to purchase and cancel the bonds on April 1, 2015, after they have been tendered by the bondholders. CL&P’s obligation to repay each series of PCRBs is secured by first mortgage bonds. Each such series of first mortgage bonds contains similar terms and provisions as the applicable series of PCRBs. If CL&P fails to meet its obligations under the first mortgage bonds, then the holder of the first mortgage bonds (the issuer of the PCRBs) would have rights under the first mortgage bonds. CL&P’s $62 million tax-exempt PCRBs, which are subject to mandatory tender for purchase on April 1, 2015 and carry a coupon rate of 1.55 percent during the current three-year fixed rate period, cannot be redeemed prior to its tender date. CL&P’s $120.5 million tax-exempt PCRBs will be subject to redemption at par on or after September 1, 2021. All other long-term debt securities are subject to make-whole provisions.
The Connecticut Light And Power Company [Member] | Series D 1994 Due 2024 7.875 % [Member]
Debt Instrument [Line Items]
Long-Term Debt	139,800,000	139,800,000
The Connecticut Light And Power Company [Member] | Series A 2004 Due 2014 4.800% [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	150,000,000
The Connecticut Light And Power Company [Member] | Series B 2004 Due 2034 5.750% [Member]
Debt Instrument [Line Items]
Long-Term Debt	130,000,000	130,000,000
The Connecticut Light And Power Company [Member] | Series A 2005 Due 2015 5.000% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
The Connecticut Light And Power Company [Member] | Series B 2005 Due 2035 5.625% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
The Connecticut Light And Power Company [Member] | Series A 2006 Due 2036 6.350% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000	250,000,000
The Connecticut Light And Power Company [Member] | Series A 2007 Due 2017 5.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000,000	150,000,000
The Connecticut Light And Power Company [Member] | Series B 2007 Due 2037 5.750% [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000,000	150,000,000
The Connecticut Light And Power Company [Member] | Series D 2007 Due 2037 6.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
The Connecticut Light And Power Company [Member] | Series A 2008 Due 2018 5.650% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000,000	300,000,000
The Connecticut Light And Power Company [Member] | Series A 2009 Due 2019 5.500% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000	250,000,000
The Connecticut Light And Power Company [Member] | One-Year Fixed Rate Tax Exempt Due 2031 [Member]
Debt Instrument [Line Items]
Long-Term Debt	62,000,000	62,000,000
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	244,500,000	244,400,000
The Connecticut Light And Power Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	2,419,800,000	2,319,800,000
The Connecticut Light And Power Company [Member] | Total Pollution Control Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	182,500,000	182,500,000
The Connecticut Light And Power Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	(4,800,000)	(5,500,000)
The Connecticut Light And Power Company [Member] | Serires C 2007 5.750% Due 2017 [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
The Connecticut Light And Power Company [Member] | Fixed Rate 4.375% due 2028 [Member]
Debt Instrument [Line Items]
Long-Term Debt	120,500,000	120,500,000
The Connecticut Light And Power Company [Member] | Series A Due 2023 2.5 percent [Member]
Debt Instrument [Line Items]
Long-Term Debt	400,000,000	400,000,000
The Connecticut Light And Power Company [Member] | Series A 2014 Due 2044 4.30% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000
NSTAR Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	4,700,000	301,650,000
Long-Term Debt	1,792,712,000	1,499,417,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	4,700,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	200,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	400,000,000
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	1,200,000,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	1,804,700,000
Long-term Debt, Description	On March 7, 2014, NSTAR Electric issued $300 million of 4.40 percent debentures, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $300 million of 4.875 percent debentures that matured on April 15, 2014. As of December 31, 2014 and 2013, the interest rate was 0.4721 percent and 0.478 percent, respectively.
NSTAR Electric Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	(7,300,000)	(5,300,000)
NSTAR Electric Company [Member] | Total Other [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,800,000,000	1,800,000,000
NSTAR Electric Company [Member] | Due 2014 4.875% [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	300,000,000
NSTAR Electric Company [Member] | Due 2022 2.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	400,000,000	400,000,000
NSTAR Electric Company [Member] | Due 2017 5.625% [Member]
Debt Instrument [Line Items]
Long-Term Debt	400,000,000	400,000,000
NSTAR Electric Company [Member] | Due 2036 5.75% [Member]
Debt Instrument [Line Items]
Long-Term Debt	200,000,000	200,000,000
NSTAR Electric Company [Member] | Due 2040 5.5% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000,000	300,000,000
NSTAR Electric Company [Member] | TaxExempt Sewage Facility Revenue Bonds Due 2015 [Member]
Debt Instrument [Line Items]
Long-Term Debt	4,700,000	6,400,000
NSTAR Electric Company [Member] | Variable Due 2016 [Member]
Debt Instrument [Line Items]
Long-Term Debt	200,000,000	200,000,000
NSTAR Electric Company [Member] | Series Due 2044 2.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000,000
Public Service Company Of New Hampshire [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	0	50,000,000
Long-Term Debt	1,076,286,000	999,006,000
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	70,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	110,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	150,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	746,300,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	1,076,300,000
Long-term Debt, Description	On July 15, 2014, PSNH repaid at maturity the $50 million of 5.25 percent Series L First Mortgage Bonds using short-term borrowings. On October 14, 2014, PSNH issued $75 million of first mortgage bonds at a yield of 3.144 percent due to mature in 2023. The first mortgage bonds are part of the same series of PSNH’s existing 3.50 percent Series S First Mortgage Bonds that were initially issued in November 2013. The proceeds, net of issuance costs, were used to repay short-term borrowings. PSNH's obligation to repay the PCRBs is secured by first mortgage bonds and bond insurance. The first mortgage bonds contain similar terms and provisions as the PCRBs. If PSNH fails to meet its obligations under the first mortgage bonds, then the holder of the first mortgage bonds (the issuer of the PCRBs) would have rights under the first mortgage bonds. The PSNH Series A tax-exempt PCRBs are currently callable at 100 percent of par. The PCRBs bear interest at a rate that is periodically set pursuant to auctions. PSNH is not obligated to purchase these PCRBs, which mature in 2021, from the remarketing agent. As of December 31, 2014 and 2013, the interest rate was 0.175 percent and 0.088 percent, respectively.
Public Service Company Of New Hampshire [Member] | Series L 2004 Due 2014 5.25% [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	50,000,000
Public Service Company Of New Hampshire [Member] | Series M 2005 Due 2035 5.60% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Public Service Company Of New Hampshire [Member] | Series N 2007 Due 2017 6.15% [Member]
Debt Instrument [Line Items]
Long-Term Debt	70,000,000	70,000,000
Public Service Company Of New Hampshire [Member] | Series O 2008 Due 2018 6.00% [Member]
Debt Instrument [Line Items]
Long-Term Debt	110,000,000	110,000,000
Public Service Company Of New Hampshire [Member] | Series P 2009 Due 2019 4.50% [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000,000	150,000,000
Public Service Company Of New Hampshire [Member] | Adjustablerateseriesadue 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	89,300,000	89,300,000
Public Service Company Of New Hampshire [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	987,000,000	962,000,000
Public Service Company Of New Hampshire [Member] | Total Pollution Control Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	89,300,000	89,300,000
Public Service Company Of New Hampshire [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	(2,300,000)
Public Service Company Of New Hampshire [Member] | Series Q 4.05% 2011 Due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	122,000,000	122,000,000
Public Service Company Of New Hampshire [Member] | Series R 2011 3.30% due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	160,000,000	160,000,000
Public Service Company Of New Hampshire [Member] | Series2013due2023member [Member]
Debt Instrument [Line Items]
Long-Term Debt	325,000,000	250,000,000
Western Massachusetts Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	50,000,000	0
Long-Term Debt	578,471,000	629,389,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	50,000,000
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	0
Long-term Debt, Maturities, Repayments of Principal after Year Five	515,000,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	565,000,000
Western Massachusetts Electric Company [Member] | Senior Notes Series B 5.90% Due 2034 [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Western Massachusetts Electric Company [Member] | Senior Notes Series C 5.24% Due 2015 [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Western Massachusetts Electric Company [Member] | Senior Notes Series D 6.70% Due 2037 [Member]
Debt Instrument [Line Items]
Long-Term Debt	40,000,000	40,000,000
Western Massachusetts Electric Company [Member] | Senior Notes Series E 5.10% Due 2020 [Member]
Debt Instrument [Line Items]
Long-Term Debt	95,000,000	95,000,000
Western Massachusetts Electric Company [Member] | Fees And Interest Due For Spent Nuclear Fuel Disposal Costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	57,400,000	57,300,000
Western Massachusetts Electric Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	6,100,000	7,100,000
Western Massachusetts Electric Company [Member] | Senior Notes Series F 3.50% due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000	250,000,000
Western Massachusetts Electric Company [Member] | Total Pollution Control Notes and Other Notes [Member]
Debt Instrument [Line Items]
Long-Term Debt	565,000,000	565,000,000
Western Massachusetts Electric Company [Member] | Series g 2013 due 2023 member [Member]
Debt Instrument [Line Items]
Long-Term Debt	80,000,000	80,000,000
Yankee Gas Services Company [Member]
Debt Instrument [Line Items]
Long-Term Debt	370,600,000	345,700,000
Yankee Gas Services Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	370,000,000	345,000,000
Yankee Gas Services Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	600,000	700,000
Yankee Gas Services Company [Member] | Series B due 2022 8.48% [Member]
Debt Instrument [Line Items]
Long-Term Debt	20,000,000	20,000,000
Yankee Gas Services Company [Member] | Series G due 2014 4.80% [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	75,000,000
Yankee Gas Services Company [Member] | Series H due 2019 5.26% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Yankee Gas Services Company [Member] | Series I due 2035 5.35% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Yankee Gas Services Company [Member] | Series J due 2018 6.90% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
Yankee Gas Services Company [Member] | Series K due 2020 4.87% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Yankee Gas Services Company [Member] | Debentures 2014 Due 2044 4.40% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000
NSTAR Gas Company [Member]
Debt Instrument [Line Items]
Long-Term Debt	210,000,000	210,000,000
NSTAR Gas Company [Member] | Series J Due 202 9.95% [Member]
Debt Instrument [Line Items]
Long-Term Debt	25,000,000	25,000,000
NSTAR Gas Company [Member] | Series K Due 2033 7.11% [Member]
Debt Instrument [Line Items]
Long-Term Debt	35,000,000	35,000,000
NSTAR Gas Company [Member] | Series M Due 2017 7.04% [Member]
Debt Instrument [Line Items]
Long-Term Debt	25,000,000	25,000,000
NSTAR Gas Company [Member] | Series N Due 2020 4.46% [Member]
Debt Instrument [Line Items]
Long-Term Debt	125,000,000	125,000,000
Subsidiaries [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	(1,200,000)	(1,300,000)
Subsidiaries [Member] | Fair Value Adjustment [Member]
Debt Instrument [Line Items]
Long-Term Debt	202,300,000	230,700,000
Subsidiaries [Member] | Total Other [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,725,700,000	1,304,400,000
Subsidiaries [Member] | Commerical Paper and Revolver Borrowings [Member]
Debt Instrument [Line Items]
Long-Term Debt	446,300,000	25,000,000
Subsidiaries [Member] | Debentures Due 2019 4.50% [Member]
Debt Instrument [Line Items]
Long-Term Debt	350,000,000	350,000,000
Subsidiaries [Member] | Spent Nuclear Fuel Obilgation CY [Member]
Debt Instrument [Line Items]
Long-Term Debt	179,400,000	179,400,000
Subsidiaries [Member] | FairValueAdjustmentMemberCurrentPortion [Member]
Debt Instrument [Line Items]
Long-Term Debt	28,900,000	31,700,000
Subsidiaries [Member] | Series F Due 2023 2.8% [Member]
Debt Instrument [Line Items]
Long-Term Debt	450,000,000	450,000,000
Subsidiaries [Member] | Series E Due 2018 1.45% [Member]
Debt Instrument [Line Items]
Long-Term Debt	$ 300,000,000	$ 300,000,000