CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS EQUITY (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Common Stock, Shares, Outstanding	316,983,337us-gaap_CommonStockSharesOutstanding	315,273,559us-gaap_CommonStockSharesOutstanding	316,983,337us-gaap_CommonStockSharesOutstanding	315,273,559us-gaap_CommonStockSharesOutstanding
Net Income	$ 223,600us-gaap_NetIncomeLoss	$ 179,200us-gaap_NetIncomeLoss	$ 827,065us-gaap_NetIncomeLoss	$ 793,689us-gaap_NetIncomeLoss	$ 533,077us-gaap_NetIncomeLoss
Other Comprehensive Income (Loss), Net of Tax			(27,978)	26,823	(2,168)
Net Income Attributable to Noncontrolling Interests			7,519	7,682	7,132
SharesIssuedInConnectionWithNstarMerger
SharesIssuedValueInConectionWithNstarMerger
Common Stock [Member]
Common Stock, Shares, Outstanding	316,983,337	315,273,559	316,983,337	315,273,559	314,053,634	177,158,692
Common Stock, Value, Outstanding	1,666,796	1,665,351	1,666,796	1,665,351	1,662,547	980,264
Issuance of Common Shares			1,445		2,040
Stock Issued During Period, Shares, New Issues			288,941	659,077	408,018
Stock Issued During Period, Shares, Treasury Stock Reissued			1,420,837		438,329
Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	6,235,834	6,192,765	6,235,834	6,192,765	6,183,267	1,797,884
Issuance of Common Shares			5,164	17,381	11,287
Long-Term Incentive Plan Activity			(9,569)	(5,409)	(3,897)
Stock Issued During Period, Value, Treasury Stock Reissued			37,817		8,454
SharesIssuedValueInConectionWithNstarMerger
OtherChangesInShareholdersEquity			9,657		8,574
OtherEquityImpactsOfMergerWithNstar
Retained Earnings [Member]
Common Stock, Value, Outstanding	2,448,661	2,125,980	2,448,661	2,125,980	1,802,714	1,651,875
Net Income			827,065	793,689	421
Dividends on Common Shares			(496,524)		(375,527)
Dividends on Preferred Stock			(7,519)		(7,029)
Net Income Attributable to Noncontrolling Interests					(103)
OtherEquityImpactsOfMergerWithNstar		(7,682)		(7,682)
Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Value, Outstanding	(74,009)	(46,031)	(74,009)	(46,031)	(72,854)	(70,686)
Other Comprehensive Income (Loss), Net of Tax			(27,978)		(2,168)
Treasury Stock [Member]
Common Stock, Value, Outstanding	(300,467)	(326,537)	(300,467)	(326,537)	(338,624)	(346,667)
Stock Issued During Period, Value, Treasury Stock Reissued			26,070		8,043
Stockholders Equity Total Member
SharesIssuedValueInConectionWithNstarMerger				(462,741)
OtherEquityImpactsOfMergerWithNstar		(7,682)		(7,682)
Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	9,976,815	9,611,528	9,976,815	9,611,528	9,237,050	4,012,670
Net Income			827,065	793,689	3,359
Dividends on Common Shares			(496,524)	11,078	(375,527)
Issuance of Common Shares			6,609	29,468	13,327
Dividends on Preferred Stock			(7,519)	(10,748)	(7,029)
Long-Term Incentive Plan Activity			(9,569)	(5,409)	(3,897)
Other Comprehensive Income (Loss), Net of Tax			(27,978)		(2,168)
Stock Issued During Period, Value, Treasury Stock Reissued			63,887	26,823	16,497
Net Income Attributable to Noncontrolling Interests					(103)
OtherChangesInShareholdersEquity			9,316		8,574
The Connecticut Light And Power Company [Member]
Net Income	87,200	60,200	287,754	279,412	209,725
Other Comprehensive Income (Loss), Net of Tax			456	413	451
Capital Contributions from NU Parent			120,000	40,000	25,000
The Connecticut Light And Power Company [Member] \| Common Stock [Member]
Common Stock, Shares, Outstanding	6,035,205	6,035,205	6,035,205	6,035,205	6,035,205	6,035,205
Common Stock, Value, Outstanding	60,352	60,352	60,352	60,352	60,352	60,352
The Connecticut Light And Power Company [Member] \| Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	1,804,869	1,682,047	1,804,869	1,682,047	1,640,149	1,613,503
Stock Issued During Period, Value, Employee Stock Ownership Plan			2,771	1,847	1,595
Payments of Stock Issuance Costs			51	51	51
Capital Contributions from NU Parent			120,000	40,000	25,000
The Connecticut Light And Power Company [Member] \| Retained Earnings [Member]
Common Stock, Value, Outstanding	1,072,477	961,482	1,072,477	961,482	839,628	735,948
Net Income			287,754	279,412	209,725
Dividends on Common Shares			(171,200)	(151,999)	(100,486)
Dividends on Preferred Stock			(5,559)	(5,559)	(5,559)
The Connecticut Light And Power Company [Member] \| Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Value, Outstanding	(931)	(1,387)	(931)	(1,387)	(1,800)	(2,251)
Other Comprehensive Income (Loss), Net of Tax			456	413
The Connecticut Light And Power Company [Member] \| Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	2,936,767	2,702,494	2,936,767	2,702,494	2,538,329	2,407,552
Net Income			287,754	279,412	209,725
Dividends on Common Shares			(171,200)	(151,999)	(100,486)
Dividends on Preferred Stock			(5,559)	(5,559)	(5,559)
Stock Issued During Period, Value, Employee Stock Ownership Plan			2,771	1,847	1,595
Other Comprehensive Income (Loss), Net of Tax			456	413
Payments of Stock Issuance Costs			51	51	51
Capital Contributions from NU Parent			120,000	40,000	25,000
NSTAR Electric Company [Member]
Net Income	69,300	55,400	303,088	268,546	190,242
Other Comprehensive Income (Loss), Net of Tax			442
NSTAR Electric Company [Member] \| Common Stock [Member]
Common Stock, Shares, Outstanding	100	100	100	100	100	100
Common Stock, Value, Outstanding	0	0	0	0	0	0
NSTAR Electric Company [Member] \| Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	994,130	992,625	994,130	992,625	992,625	992,625
NSTAR Electric Company [Member] \| Retained Earnings [Member]
Common Stock, Value, Outstanding	1,468,955	1,420,828	1,468,955	1,420,828	1,210,405	1,239,123
Net Income				268,546	190,242
Dividends on Common Shares			(253,000)	(56,000)	(217,000)
Dividends on Preferred Stock			(1,961)	(2,123)	(1,960)
NSTAR Electric Company [Member] \| Accumulated Other Comprehensive Income (Loss) [Member]
Other Comprehensive Income (Loss), Net of Tax			442
NSTAR Electric Company [Member] \| Stockholders Equity Total Member
Other Comprehensive Income (Loss), Net of Tax			442
NSTAR Electric Company [Member] \| Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	2,463,527	2,413,453	2,463,527	2,413,453	2,203,030	2,231,748
Net Income			303,088	268,546	190,242
Dividends on Common Shares			(253,000)	(56,000)	(217,000)
Dividends on Preferred Stock			(1,961)	(2,123)	(1,960)
Public Service Company Of New Hampshire [Member]
Net Income	29,000	26,800	113,944	111,397	96,882
Other Comprehensive Income (Loss), Net of Tax			1,181	1,105	1,177
Capital Contributions from NU Parent			45,000	0	0
Public Service Company Of New Hampshire [Member] \| Common Stock [Member]
Common Stock, Shares, Outstanding	301	301	301	301	301	301
Common Stock, Value, Outstanding	0	0	0	0	0	0
Public Service Company Of New Hampshire [Member] \| Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	748,240	701,911	748,240	701,911	701,052	700,285
Stock Issued During Period, Value, Employee Stock Ownership Plan			1,329	859	767
Public Service Company Of New Hampshire [Member] \| Retained Earnings [Member]
Common Stock, Value, Outstanding	486,459	438,515	486,459	438,515	395,118	388,910
Net Income			113,944	111,397	96,882
Dividends on Common Shares			(66,000)	(68,000)	(90,674)
Public Service Company Of New Hampshire [Member] \| Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Value, Outstanding	(7,369)	(8,550)	(7,369)	(8,550)	(9,655)	(10,832)
Other Comprehensive Income (Loss), Net of Tax			1,181	1,105	1,177
Public Service Company Of New Hampshire [Member] \| Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	1,227,330	1,131,876	1,227,330	1,131,876	1,086,515	1,078,363
Net Income			113,944	111,397	96,882
Dividends on Common Shares			(66,000)	(68,000)	(90,674)
Stock Issued During Period, Value, Employee Stock Ownership Plan			1,329	859	767
Other Comprehensive Income (Loss), Net of Tax			1,181	1,105	1,177
Western Massachusetts Electric Company [Member]
Net Income	18,000	10,400	57,819	60,438	54,503
Other Comprehensive Income (Loss), Net of Tax			341	329	340
Capital Contributions from NU Parent			0	0	50,000
Western Massachusetts Electric Company [Member] \| Common Stock [Member]
Common Stock, Shares, Outstanding	434,653	434,653	434,653	434,653	434,653	434,653
Common Stock, Value, Outstanding	10,866	10,866	10,866	10,866	10,866	10,866
Western Massachusetts Electric Company [Member] \| Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	391,256	390,743	391,256	390,743	390,412	340,115
Stock Issued During Period, Value, Employee Stock Ownership Plan			513	331	297
Capital Contributions from NU Parent					50,000
Western Massachusetts Electric Company [Member] \| Retained Earnings [Member]
Common Stock, Value, Outstanding	178,834	181,014	178,834	181,014	160,577	115,506
Net Income			57,819	60,438	54,503
Dividends on Common Shares			(59,999)	(40,001)	(9,432)
Western Massachusetts Electric Company [Member] \| Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Value, Outstanding	(3,176)	(3,517)	(3,176)	(3,517)	(3,846)	(4,186)
Other Comprehensive Income (Loss), Net of Tax			341		340
Western Massachusetts Electric Company [Member] \| Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	577,780	579,106	577,780	579,106	558,009	462,301
Net Income			57,819	60,438	54,503
Dividends on Common Shares			(59,999)	(40,001)	(9,432)
Stock Issued During Period, Value, Employee Stock Ownership Plan			513	331	297
Other Comprehensive Income (Loss), Net of Tax			341		340
Capital Contributions from NU Parent				$ 329	$ 50,000

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