CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS EQUITY (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended								12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended								12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended								12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended												12 Months Ended				12 Months Ended
	Dec. 31, 2012 Common Stock [Member]	Dec. 31, 2011 Common Stock [Member]	Dec. 31, 2010 Common Stock [Member]	Dec. 31, 2009 Common Stock [Member]	Dec. 31, 2012 Additional Paid-in Capital [Member]	Dec. 31, 2011 Additional Paid-in Capital [Member]	Dec. 31, 2010 Additional Paid-in Capital [Member]	Dec. 31, 2009 Additional Paid-in Capital [Member]	Dec. 31, 2010 Deferred Contribution Plan [Member]	Dec. 31, 2012 Deferred Contribution Plan [Member]	Dec. 31, 2011 Deferred Contribution Plan [Member]	Dec. 31, 2009 Deferred Contribution Plan [Member]	Dec. 31, 2012 Retained Earnings [Member]	Dec. 31, 2011 Retained Earnings [Member]	Dec. 31, 2010 Retained Earnings [Member]	Dec. 31, 2009 Retained Earnings [Member]	Dec. 31, 2012 Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2011 Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2010 Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2009 Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2012 Treasury Stock [Member]	Dec. 31, 2011 Treasury Stock [Member]	Dec. 31, 2010 Treasury Stock [Member]	Dec. 31, 2009 Treasury Stock [Member]	Dec. 31, 2012 Common Shareholders Equity [Member]	Dec. 31, 2011 Common Shareholders Equity [Member]	Dec. 31, 2010 Common Shareholders Equity [Member]	Dec. 31, 2009 Common Shareholders Equity [Member]	Dec. 31, 2012 The Connecticut Light And Power Company [Member] Common Stock [Member]	Dec. 31, 2011 The Connecticut Light And Power Company [Member] Common Stock [Member]	Dec. 31, 2010 The Connecticut Light And Power Company [Member] Common Stock [Member]	Dec. 31, 2009 The Connecticut Light And Power Company [Member] Common Stock [Member]	Dec. 31, 2012 The Connecticut Light And Power Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2011 The Connecticut Light And Power Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2010 The Connecticut Light And Power Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2009 The Connecticut Light And Power Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2012 The Connecticut Light And Power Company [Member] Retained Earnings [Member]	Dec. 31, 2011 The Connecticut Light And Power Company [Member] Retained Earnings [Member]	Dec. 31, 2010 The Connecticut Light And Power Company [Member] Retained Earnings [Member]	Dec. 31, 2009 The Connecticut Light And Power Company [Member] Retained Earnings [Member]	Dec. 31, 2012 The Connecticut Light And Power Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2011 The Connecticut Light And Power Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2010 The Connecticut Light And Power Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2009 The Connecticut Light And Power Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2012 The Connecticut Light And Power Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2011 The Connecticut Light And Power Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2010 The Connecticut Light And Power Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2009 The Connecticut Light And Power Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2012 Public Service Company Of New Hampshire [Member] Common Stock [Member]	Dec. 31, 2011 Public Service Company Of New Hampshire [Member] Common Stock [Member]	Dec. 31, 2010 Public Service Company Of New Hampshire [Member] Common Stock [Member]	Dec. 31, 2009 Public Service Company Of New Hampshire [Member] Common Stock [Member]	Dec. 31, 2012 Public Service Company Of New Hampshire [Member] Additional Paid-in Capital [Member]	Dec. 31, 2011 Public Service Company Of New Hampshire [Member] Additional Paid-in Capital [Member]	Dec. 31, 2010 Public Service Company Of New Hampshire [Member] Additional Paid-in Capital [Member]	Dec. 31, 2009 Public Service Company Of New Hampshire [Member] Additional Paid-in Capital [Member]	Dec. 31, 2012 Public Service Company Of New Hampshire [Member] Retained Earnings [Member]	Dec. 31, 2011 Public Service Company Of New Hampshire [Member] Retained Earnings [Member]	Dec. 31, 2010 Public Service Company Of New Hampshire [Member] Retained Earnings [Member]	Dec. 31, 2009 Public Service Company Of New Hampshire [Member] Retained Earnings [Member]	Dec. 31, 2012 Public Service Company Of New Hampshire [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2011 Public Service Company Of New Hampshire [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2010 Public Service Company Of New Hampshire [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2009 Public Service Company Of New Hampshire [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2012 Public Service Company Of New Hampshire [Member] Common Shareholders Equity [Member]	Dec. 31, 2011 Public Service Company Of New Hampshire [Member] Common Shareholders Equity [Member]	Dec. 31, 2010 Public Service Company Of New Hampshire [Member] Common Shareholders Equity [Member]	Dec. 31, 2009 Public Service Company Of New Hampshire [Member] Common Shareholders Equity [Member]	Dec. 31, 2012 Western Massachusetts Electric Company [Member] Common Stock [Member]	Dec. 31, 2011 Western Massachusetts Electric Company [Member] Common Stock [Member]	Dec. 31, 2010 Western Massachusetts Electric Company [Member] Common Stock [Member]	Dec. 31, 2009 Western Massachusetts Electric Company [Member] Common Stock [Member]	Dec. 31, 2012 Western Massachusetts Electric Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2011 Western Massachusetts Electric Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2010 Western Massachusetts Electric Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2009 Western Massachusetts Electric Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2012 Western Massachusetts Electric Company [Member] Retained Earnings [Member]	Dec. 31, 2011 Western Massachusetts Electric Company [Member] Retained Earnings [Member]	Dec. 31, 2010 Western Massachusetts Electric Company [Member] Retained Earnings [Member]	Dec. 31, 2009 Western Massachusetts Electric Company [Member] Retained Earnings [Member]	Dec. 31, 2012 Western Massachusetts Electric Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2011 Western Massachusetts Electric Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2010 Western Massachusetts Electric Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2009 Western Massachusetts Electric Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2012 Western Massachusetts Electric Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2011 Western Massachusetts Electric Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2010 Western Massachusetts Electric Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2009 Western Massachusetts Electric Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2012 NSTAR Electric Company [Member] Common Stock [Member]	Dec. 31, 2011 NSTAR Electric Company [Member] Common Stock [Member]	Dec. 31, 2010 NSTAR Electric Company [Member] Common Stock [Member]	Dec. 31, 2009 NSTAR Electric Company [Member] Common Stock [Member]	Dec. 31, 2012 NSTAR Electric Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2011 NSTAR Electric Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2010 NSTAR Electric Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2009 NSTAR Electric Company [Member] Additional Paid-in Capital [Member]	Dec. 31, 2012 NSTAR Electric Company [Member] Retained Earnings [Member]	Dec. 31, 2011 NSTAR Electric Company [Member] Retained Earnings [Member]	Dec. 31, 2010 NSTAR Electric Company [Member] Retained Earnings [Member]	Dec. 31, 2009 NSTAR Electric Company [Member] Retained Earnings [Member]	Dec. 31, 2012 NSTAR Electric Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2011 NSTAR Electric Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2010 NSTAR Electric Company [Member] Common Shareholders Equity [Member]	Dec. 31, 2009 NSTAR Electric Company [Member] Common Shareholders Equity [Member]
Common Stock, Shares, Outstanding	314,053,634	177,158,692	176,448,081	175,620,024																									6,035,205	6,035,205	6,035,205	6,035,205																	301	301	301	301																	434,653	434,653	434,653	434,653																	100	100	100	100
Common Stock, Value, Outstanding	$ 1,662,547	$ 980,264	$ 978,909	$ 977,276	$ 6,183,267	$ 1,797,884	$ 1,777,592	$ 1,762,097	$ 0	$ 0	$ 0	$ (2,944)	$ 1,802,714	$ 1,651,875	$ 1,452,777	$ 1,246,543	$ (72,854)	$ (70,686)	$ (43,370)	$ (43,467)	$ (338,624)	$ (346,667)	$ (354,732)	$ (361,603)	$ 9,237,050	$ 4,012,670	$ 3,811,176	$ 3,577,902	$ 60,352	$ 60,352	$ 60,352	$ 60,352	$ 1,640,149	$ 1,613,503	$ 1,605,275	$ 1,601,792	$ 839,628	$ 735,948	$ 734,561	$ 714,210	$ (1,800)	$ (2,251)	$ (2,713)	$ (3,171)	$ 2,538,329	$ 2,407,552	$ 2,397,475	$ 2,373,183	$ 0	$ 0	$ 0	$ 0	$ 701,052	$ 700,285	$ 579,577	$ 420,169	$ 395,118	$ 388,910	$ 347,471	$ 307,988	$ (9,655)	$ (10,832)	$ (601)	$ (712)	$ 1,086,515	$ 1,078,363	$ 926,447	$ 727,445	$ 10,866	$ 10,866	$ 10,866	$ 10,866	$ 390,412	$ 340,115	$ 248,044	$ 145,400	$ 160,577	$ 115,506	$ 98,757	$ 90,549	$ (3,846)	$ (4,186)	$ (83)	$ (8)	$ 558,009	$ 462,301	$ 357,584	$ 246,807	$ 0	$ 0	$ 0	$ 0	$ 992,625	$ 992,625	$ 992,625	$ 992,625	$ 1,210,405	$ 1,239,123	$ 1,158,489	$ 1,100,074	$ 2,203,030	$ 2,231,748	$ 2,151,114	$ 2,092,699
Net Income													533,077	400,513	394,107										533,077	400,513	394,107										209,725	250,164	244,143						209,725	250,164	244,143										96,882	100,267	90,067						96,882	100,267	90,067										54,503	43,054	23,090						54,503	43,054	23,090										190,242	252,494	248,575		190,242	252,494	248,575
Dividends on Common Shares													(375,527)	(195,595)	(181,715)										(375,527)	(195,595)	(181,715)										(100,486)	(243,218)	(217,691)						(100,486)	(243,218)	(217,691)										(90,674)	(58,828)	(50,584)						(90,674)	(58,828)	(50,584)										(9,432)	(26,305)	(14,882)						(9,432)	(26,305)	(14,882)										(217,000)	(169,900)	(188,200)		(217,000)	(169,900)	(188,200)
Issuance of Common Shares	2,040	1,355	1,633		11,287	4,496	5,745																		13,327	5,851	7,378
Stock Issued During Period, Shares, New Issues	408,018	271,030	326,526
Dividends, Preferred Stock													(7,029)	(5,559)	(6,101)										(7,029)	(5,559)	(6,101)										(5,559)	(5,559)	(6,101)						(5,559)	(5,559)	(6,101)																																																		(1,960)	(1,960)	(1,960)		(1,960)	(1,960)	(1,960)
Stock Issued During Period, Value, Employee Stock Ownership Plan							439		2,944																		3,383						1,595	1,429	919										1,595	1,429	919						767	678	439										767	678	439						297	259	165										297	259	165
Stock Issued During Period, Shares, Employee Stock Ownership Plan			127,054
Long-Term Incentive Plan Activity					(3,897)	7,359	4,868																		(3,897)	7,359	4,868
Other Comprehensive Income (Loss), Net of Tax																	(2,168)	(27,316)	97						(2,168)	(27,316)	97														451	462	458		451	462	458														1,177	(10,231)	111		1,177	(10,231)	111														340	(4,103)	(75)		340	(4,103)	(75)
Stock Issued During Period, Value, Treasury Stock Reissued					8,454	7,048	3,856														8,043	8,065	6,871		16,497	15,113	10,727
Stock Issued During Period, Shares, Treasury Stock Reissued	438,329	439,581	374,477
Payments of Stock Issuance Costs																																	51	51	51										51	51	51
Net Income Attributable to Noncontrolling Interests													(103)	(261)	(57)										(103)	(261)	(57)
Capital Contributions from NU Parent																																	25,000	6,748	2,513										25,000	6,748	2,513							120,030	158,969											120,030	158,969						50,000	91,812	102,479										50,000	91,812	102,479
SharesIssuedInConnectionWithNstarMerger	136,048,595
SharesIssuedValueInConectionWithNstarMerger	680,243				4,358,027																				5,038,270
OtherChangesInShareholdersEquity					8,574	1,389	587																		8,574	1,389	587
OtherEquityImpactsOfMergerWithNstar					$ 2,938								$ 421												$ 3,359