LONG-TERM DEBT (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	$ 763,338,000	$ 331,582,000
Long-Term Debt	7,200,156,000	4,614,913,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	731,700,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	576,600,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	216,700,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	0
Long-term Debt, Maturities, Repayments of Principal in Year Five	745,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	4,559,800,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	6,829,800,000
Long-term Debt, Description	On March 22, 2012, NU parent issued $300 million of floating rate Series D Senior Notes with a maturity date of September 20, 2013. The notes have a coupon rate based on the three-month LIBOR rate plus a credit spread of 0.75 percent and will reset quarterly. The notes had an interest rate of 1.059 percent as of December 31, 2012. The proceeds, net of issuance expenses, were used to repay at maturity the NU parent $263 million Series A Senior Notes that matured on April 1, 2012, to repay short-term borrowings outstanding under the NU parent Credit Agreement and for other general corporate purposes.          As of December 31, 2012, amount relates to the purchase price adjustment required to record the NSTAR long-term debt issuances at fair value on the date of the merger. As of December 31, 2011, amount related to a fixed to floating interest rate swap on the $263 million NU parent note that matured on April 1, 2012. The change in fair value of the interest component of the debt was recorded as an adjustment to Current Portion - Long Term Debt as of December 31, 2011 with an equal and offsetting adjustment to Current Derivative Assets. . Fees due to the DOE for the disposal of CL&P's and WMECO's Prior Period Spent Nuclear Fuel and CYAPC's and YAEC's spent nuclear fuel obligation include accumulated interest costs of $350 million and $219.3 million for NU ($177.8 million and $177.6 million for CL&P and $41.7 million and $41.7 million for WMECO) as of December 31, 2012 and 2011, respectively.
Total Other [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,079,300,000	513,000,000
Series A due 2012 7.25% [Member]
Debt Instrument [Line Items]
Long-Term Debt		263,000,000
Series C due 2013 5.65% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000	250,000,000
Debentures Due 2019 4.50% [Member]
Debt Instrument [Line Items]
Long-Term Debt	350,000,000
Spent Nuclear Fuel Obilgation CY [Member]
Debt Instrument [Line Items]
Long-Term Debt	179,300,000
Public Service Company Of New Hampshire [Member]
Debt Instrument [Line Items]
Long-Term Debt	997,932,000	997,722,000
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	50,000,000
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	70,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	879,500,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	999,500,000
Long-term Debt, Description	The PSNH Series A and Series C tax-exempt bonds are currently callable at 100 percent and 101 percent of par, respectively. The PSNH Series B tax-exempt bond will become callable in June 2013. CL&P’s $125 million and $62 million tax-exempt PCRBs, which are subject to mandatory tender for purchase on September 3, 2013 and April 1, 2015, respectively, cannot be redeemed prior to their respective tender dates. 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. As of December 31, 2012, PSNH had $287.5 million in PCRBs outstanding. PSNH's obligation to repay each series of PCRBs is secured by first mortgage bonds and bond insurance. Each such series of first mortgage bonds contains similar terms and provisions as the applicable series of PCRBs. If PSNH failed to meet its obligations under the PCRBs, then these first mortgage bonds would become outstanding. The 2001 Series A PCRBs, in the aggregate principal amount of $89.3 million, bears 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. The weighted average effective interest rate on PSNH's Series A variable-rate PCRBs was 0.20 percent in 2012 and 0.21 percent in 2011.
Public Service Company Of New Hampshire [Member] | Series L 2004 Due 2014 5.25% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	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	712,000,000	712,000,000
Public Service Company Of New Hampshire [Member] | Total Pollution Control Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	287,500,000
Public Service Company Of New Hampshire [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	(1,600,000)	(1,800,000)
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] | Total Pollution Control Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt		287,500,000
Public Service Company Of New Hampshire [Member] | Tax Exempt Series B and C Due 2021 4.75% to 5.45% [Member]
Debt Instrument [Line Items]
Long-Term Debt	198,200,000	198,200,000
The Connecticut Light And Power Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	125,000,000	62,000,000
Long-Term Debt	2,737,790,000	2,521,753,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	125,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	150,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	162,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	0
Long-term Debt, Maturities, Repayments of Principal in Year Five	250,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,540,300,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	2,227,300,000
Long-term Debt, Description	On October 1, 2012, CL&P redeemed at par four different series of tax-exempt PCRBs totaling $116.4 million. The PCRBs had maturity dates ranging from 2016 through 2028 and coupon rates of 5.85 percent through 5.95 percent. The $125 million of tax-exempt PCRBs were issued with an initial fixed rate term period ending on September 2, 2013, and are subject to mandatory tender for purchase on September 3, 2013, at which time CL&P expects to remarket the PCRBs. On April 2, 2012, CL&P remarketed $62 million of tax-exempt PCRBs for a three-year period. The PCRBs, which mature on May 1, 2031, carry a coupon rate of 1.55 percent during the current three-year fixed rate period and are subject to mandatory tender for purchase on April 1, 2015. On January 15, 2013, CL&P issued $400 million of 2.5 percent Series A First and Refunding Mortgage Bonds with a maturity date of January 15, 2023. The proceeds, net of issuance expenses, were used to repay the amounts outstanding under the CL&P revolver and the NU commercial paper program. As a result, these amounts have been classified as Long-Term Debt as of December 31, 2012. As of December 31, 2012, CL&P had $307.5 million of tax-exempt PCRBs outstanding. 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 failed to meet its obligations under the PCRBs, then these first mortgage bonds would become outstanding
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	150,000,000	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] | Fixed Rate 5.85% To 5.90% Due 2016 To 2022 [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	116,400,000
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	244,300,000	244,100,000
The Connecticut Light And Power Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,919,800,000	1,919,800,000
The Connecticut Light And Power Company [Member] | Total Pollution Control Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	307,500,000	423,900,000
The Connecticut Light And Power Company [Member] | Total First Mortgage Bonds and Pollution Control Notes [Member]
Debt Instrument [Line Items]
Long-Term Debt	2,227,300,000	2,343,700,000
The Connecticut Light And Power Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	(3,600,000)	(4,000,000)
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] | Fixed Rate 1.25% due 2028 [Member]
Debt Instrument [Line Items]
Long-Term Debt	125,000,000	125,000,000
The Connecticut Light And Power Company [Member] | Commerical Paper and Revolver Borrowings [Member]
Debt Instrument [Line Items]
Long-Term Debt	394,800,000
Western Massachusetts Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	55,000,000	0
Long-Term Debt	550,270,000	499,545,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	55,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	0
Long-term Debt, Maturities, Repayments of Principal in Year Three	50,000,000
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	435,000,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	540,000,000
Long-term Debt, Description	On October 1, 2012, WMECO redeemed at par $53.8 million of tax-exempt PCRBs. The PCRBs had a maturity date of 2028 and a coupon of 5.85 percent. On October 4, 2012, WMECO issued at a premium $150 million of senior unsecured notes at a yield of 2.673 percent that will mature on September 15, 2021. The senior notes are part of the same series of WMECO’s existing 3.5 percent coupon Series F Senior Notes that were initially issued in September 2011. As a result, the aggregate principal amount of WMECO’s outstanding Series F Senior Notes totaled $250 million.
Western Massachusetts Electric Company [Member] | Tax Exempt 1993 Series A 5.85% Due 2028 [Member]
Debt Instrument [Line Items]
Long-Term Debt		53,800,000
Western Massachusetts Electric Company [Member] | Senior Note Series A 5.00% Due 2013 [Member]
Debt Instrument [Line Items]
Long-Term Debt	55,000,000	55,000,000
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	57,300,000	57,300,000
Western Massachusetts Electric Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	8,000,000	(1,600,000)
Western Massachusetts Electric Company [Member] | Senior Notes Series F 3.50% due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000	100,000,000
Western Massachusetts Electric Company [Member] | Total Pollution Control Notes and Other Notes [Member]
Debt Instrument [Line Items]
Long-Term Debt	540,000,000	443,800,000
Subsidiaries [Member] | Fair Value Adjustment [Member]
Debt Instrument [Line Items]
Long-Term Debt	259,900,000	2,300,000
Subsidiaries [Member] | FairValueAdjustmentMemberCurrentPortion [Member]
Debt Instrument [Line Items]
Long-Term Debt	31,700,000	2,300,000
Subsidiaries [Member] | Variable Rates Series D Due 2013 [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000,000
Yankee Gas Services Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	345,000,000	349,300,000
Yankee Gas Services Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	800,000	900,000
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 E due 2012 7.19% [Member]
Debt Instrument [Line Items]
Long-Term Debt		4,300,000
Yankee Gas Services Company [Member] | Series G due 2014 4.80% [Member]
Debt Instrument [Line Items]
Long-Term Debt	75,000,000	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
NSTAR Gas Company [Member] | Total First Mortgage Bonds and Pollution Control Notes [Member]
Debt Instrument [Line Items]
Long-Term Debt	210,000,000
NSTAR Gas Company [Member] | Series J Due 202 9.95% [Member]
Debt Instrument [Line Items]
Long-Term Debt	25,000,000
NSTAR Gas Company [Member] | Series K Due 2033 7.11% [Member]
Debt Instrument [Line Items]
Long-Term Debt	35,000,000
NSTAR Gas Company [Member] | Series M Due 2017 7.04% [Member]
Debt Instrument [Line Items]
Long-Term Debt	25,000,000
NSTAR Gas Company [Member] | Series N Due 2020 4.46% [Member]
Debt Instrument [Line Items]
Long-Term Debt	125,000,000
NSTAR Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	1,650,000	401,650,000
Long-Term Debt	1,600,911,000	1,203,344,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Description	On October 15, 2012, NSTAR Electric issued at a discount $400 million of 2.375 percent Debentures at a yield of 2.406 percent that will mature on October 15, 2022. The proceeds, net of issuance costs, were used to pay $400 million of 4.875 percent Debentures that matured on October 15, 2012.
NSTAR Electric Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	(5,400,000)	(4,700,000)
NSTAR Electric Company [Member] | Total Other [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,600,000,000	1,600,000,000
NSTAR Electric Company [Member] | Due 2012 4.875% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000,000	400,000,000
NSTAR Electric Company [Member] | Due 2014 4.875% [Member]
Debt Instrument [Line Items]
Long-Term Debt		300,000,000
NSTAR Electric Company [Member] | Due 2022 2.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	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	$ 8,000,000	$ 8,700,000