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
	Dec. 31, 2011 Common Stock [Member]	Dec. 31, 2010 Common Stock [Member]	Dec. 31, 2009 Common Stock [Member]	Dec. 31, 2008 Common Stock [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, 2008 Additional Paid-in Capital [Member]	Dec. 31, 2010 Deferred Contribution Plan [Member]	Dec. 31, 2009 Deferred Contribution Plan [Member]	Dec. 31, 2011 Deferred Contribution Plan [Member]	Dec. 31, 2008 Deferred Contribution Plan [Member]	Dec. 31, 2011 Retained Earnings [Member]	Dec. 31, 2010 Retained Earnings [Member]	Dec. 31, 2009 Retained Earnings [Member]	Dec. 31, 2008 Retained Earnings [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, 2008 Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2011 Treasury Stock [Member]	Dec. 31, 2010 Treasury Stock [Member]	Dec. 31, 2009 Treasury Stock [Member]	Dec. 31, 2008 Treasury Stock [Member]	Dec. 31, 2011 Common Shareholders Equity [Member]	Dec. 31, 2010 Common Shareholders Equity [Member]	Dec. 31, 2009 Common Shareholders Equity [Member]	Dec. 31, 2008 Common Shareholders Equity [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, 2008 The Connecticut Light And Power Company [Member] Common Stock [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, 2008 The Connecticut Light And Power Company [Member] Additional Paid-in Capital [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, 2008 The Connecticut Light And Power Company [Member] Retained Earnings [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, 2008 The Connecticut Light And Power Company [Member] Accumulated Other Comprehensive Income (Loss) [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, 2008 The Connecticut Light And Power Company [Member] Common Shareholders Equity [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, 2008 Public Service Company Of New Hampshire [Member] Common Stock [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, 2008 Public Service Company Of New Hampshire [Member] Additional Paid-in Capital [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, 2008 Public Service Company Of New Hampshire [Member] Retained Earnings [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, 2008 Public Service Company Of New Hampshire [Member] Accumulated Other Comprehensive Income (Loss) [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, 2008 Public Service Company Of New Hampshire [Member] Common Shareholders Equity [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, 2008 Western Massachusetts Electric Company [Member] Common Stock [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, 2008 Western Massachusetts Electric Company [Member] Additional Paid-in Capital [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, 2008 Western Massachusetts Electric Company [Member] Retained Earnings [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, 2008 Western Massachusetts Electric Company [Member] Accumulated Other Comprehensive Income (Loss) [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, 2008 Western Massachusetts Electric Company [Member] Common Shareholders Equity [Member]
Common Stock, Shares, Outstanding	177,158,692	176,448,081	175,620,024	155,834,361																									6,035,205	6,035,205	6,035,205	6,035,205																	301	301	301	301																	434,653	434,653	434,653	434,653
Common Stock, Value, Outstanding	$ 980,264	$ 978,909	$ 977,276	$ 881,061	$ 1,797,884	$ 1,777,592	$ 1,762,097	$ 1,475,006	$ 0	$ (2,944)	$ 0	$ (15,481)	$ 1,651,875	$ 1,452,777	$ 1,246,543	$ 1,078,594	$ (70,686)	$ (43,370)	$ (43,467)	$ (37,265)	$ (346,667)	$ (354,732)	$ (361,603)	$ (361,603)	$ 4,012,670	$ 3,811,176	$ 3,577,902	$ 3,020,312	$ 60,352	$ 60,352	$ 60,352	$ 60,352	$ 1,613,503	$ 1,605,275	$ 1,601,792	$ 1,454,198	$ 735,948	$ 734,561	$ 714,210	$ 617,276	$ (2,251)	$ (2,713)	$ (3,171)	$ (3,586)	$ 2,407,552	$ 2,397,475	$ 2,373,183	$ 2,128,240	$ 0	$ 0	$ 0	$ 0	$ 700,285	$ 579,577	$ 420,169	$ 351,245	$ 388,910	$ 347,471	$ 307,988	$ 283,219	$ (10,832)	$ (601)	$ (712)	$ (749)	$ 1,078,363	$ 926,447	$ 727,445	$ 633,715	$ 10,866	$ 10,866	$ 10,866	$ 10,866	$ 340,115	$ 248,044	$ 145,400	$ 144,545	$ 115,506	$ 98,757	$ 90,549	$ 82,549	$ (4,186)	$ (83)	$ (8)	$ 190	$ 462,301	$ 357,584	$ 246,807	$ 238,150
Adoption Of Accounting Guidance For Other Than Temporary Impairments															728				(728)								0												25				(25)				0												43				(43)				0												7				(7)				0
Net Income													400,513	394,107	335,592										400,513	394,107	335,592										250,164	244,143	216,316						250,164	244,143	216,316										100,267	90,067	65,570						100,267	90,067	65,570										43,054	23,090	26,196						43,054	23,090	26,196
Dividends on Common Shares													(195,595)	(181,715)	(162,812)										(195,595)	(181,715)	(162,812)										(243,218)	(217,691)	(113,848)						(243,218)	(217,691)	(113,848)										(58,828)	(50,584)	(40,844)						(58,828)	(50,584)	(40,844)										(26,305)	(14,882)	(18,203)						(26,305)	(14,882)	(18,203)
Issuance of Common Shares	1,355	1,633	96,215		4,496	5,745	293,502																		5,851	7,378	389,717
Stock Issued During Period, Shares, New Issues	271,030	326,526	19,242,939
Dividends, Preferred Stock													(5,559)	(6,101)	(5,559)										(5,559)	(6,101)	(5,559)										(5,559)	(6,101)	(5,559)						(5,559)	(6,101)	(5,559)
Stock Issued During Period, Value, Employee Stock Ownership Plan						439	(98)		2,944	12,537																3,383	12,439						1,429	919	(48)										1,429	919	(48)						678	439	(22)										678	439	(22)						259	165	(8)										259	165	(8)
Stock Issued During Period, Shares, Employee Stock Ownership Plan		127,054	542,724
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures					7,359	4,868	5,303																		7,359	4,868	5,303
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation					1,338	866	913																		1,338	866	913
Other Comprehensive Income/ (Loss)																	(27,316)	97	(5,474)						(27,316)	97	(5,474)														462	458	440		462	458	440														(10,231)	111	80		(10,231)	111	80														(4,103)	(75)	(191)		(4,103)	(75)	(191)
Stock Issued During Period, Value, Treasury Stock Reissued																					15,113	10,727			15,113	10,727
Stock Issued During Period, Shares, Treasury Stock Reissued	439,581	374,477
Payments of Stock Issuance Costs						(279)	(12,529)																			(279)	(12,529)						51	51	51										51	51	51
ESOP Benefits From Treasury Shares					7,099	3,856															(7,048)	(3,856)			51	0
Net Income Attributable to Noncontrolling Interests													(261)	(57)											(261)	(57)
Capital Contributions from NU Parent																																	$ 6,748	$ 2,513	$ 147,591										$ 6,748	$ 2,513	$ 147,591						$ 120,030	$ 158,969	$ 68,946										$ 120,030	$ 158,969	$ 68,946						$ 91,812	$ 102,479	$ 863										$ 91,812	$ 102,479	$ 863