CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Revenues	$ 6,273,787	$ 4,465,657	$ 4,898,167
Operating Expenses:
Purchased Power and Transmission	2,084,364	1,657,914	2,034,501
Operations and Maintenance	1,583,070	1,095,358	1,001,349
Depreciation	519,010	302,192	300,737
Amortization of Regulatory Assets/(Liabilities), Net	79,762	91,080	90,054
Amortization of Rate Reduction Bonds	142,019	69,912	232,871
Energy Efficiency Programs	313,149	131,415	124,023
Taxes Other Than Income Taxes	434,207	323,610	314,741
Total Operating Expenses	5,155,581	3,671,481	4,098,276
Operating Income	1,118,206	794,176	799,891
Interest Expense:
Interest on Long-Term Debt	316,987	231,630	231,089
Interest on Rate Reduction Bonds	6,168	8,611	20,573
Other Interest	6,790	10,184	(14,371)
Total Interest Expense	329,945	250,425	237,291
Other Income/(Loss), Net	19,742	27,715	41,916
Income Before Income Tax Expense	808,003	571,466	604,516
Income Tax Expense	274,926	170,953	210,409
Net Income	533,077	400,513	394,107
Net Income Attributable to Noncontrolling Interests	7,132	5,820	6,158
Net Income Attributable to Controlling Interests	525,945	394,693	387,949
Diluted Earnings Per Common Share	$ 1.89	$ 2.22	$ 2.19
Basic Earnings Per Common Share	$ 1.90	$ 2.22	$ 2.20
Weighted Average Common Shares Outstanding:
Basic	277,209,819	177,410,167	176,636,086
Diluted	277,993,631	177,804,568	176,885,387
The Connecticut Light And Power Company [Member]
Operating Revenues	2,407,449	2,548,387	2,999,102
Operating Expenses:
Purchased Power and Transmission	858,231	982,514	1,292,733
Operations and Maintenance	635,733	580,736	494,203
Depreciation	166,853	157,747	172,167
Amortization of Regulatory Assets/(Liabilities), Net	14,372	61,025	78,870
Amortization of Rate Reduction Bonds	0	0	167,021
Energy Efficiency Programs	89,299	90,297	92,279
Taxes Other Than Income Taxes	215,972	212,885	214,179
Total Operating Expenses	1,980,460	2,085,204	2,511,452
Operating Income	426,989	463,183	487,650
Interest Expense:
Interest on Long-Term Debt	124,894	131,918	134,553
Interest on Rate Reduction Bonds	0	0	7,542
Other Interest	8,233	809	(4,357)
Total Interest Expense	133,127	132,727	137,738
Other Income/(Loss), Net	10,300	9,741	26,669
Income Before Income Tax Expense	304,162	340,197	376,581
Income Tax Expense	94,437	90,033	132,438
Net Income	209,725	250,164	244,143
Public Service Company Of New Hampshire [Member]
Operating Revenues	988,013	1,013,003	1,033,439
Operating Expenses:
Purchased Power and Transmission	319,253	327,905	389,538
Operations and Maintenance	263,234	278,153	274,165
Depreciation	87,602	76,167	67,237
Amortization of Regulatory Assets/(Liabilities), Net	(24,086)	25,383	11,232
Amortization of Rate Reduction Bonds	56,645	53,389	50,357
Energy Efficiency Programs	14,245	12,917	12,038
Taxes Other Than Income Taxes	66,025	58,985	52,686
Total Operating Expenses	782,918	832,899	857,253
Operating Income	205,095	180,104	176,186
Interest Expense:
Interest on Long-Term Debt	46,228	36,832	36,220
Interest on Rate Reduction Bonds	2,687	6,276	9,660
Other Interest	1,313	1,039	1,187
Total Interest Expense	50,228	44,147	47,067
Other Income/(Loss), Net	3,008	14,255	11,749
Income Before Income Tax Expense	157,875	150,212	140,868
Income Tax Expense	60,993	49,945	50,801
Net Income	96,882	100,267	90,067
Western Massachusetts Electric Company [Member]
Operating Revenues	441,164	417,315	395,161
Operating Expenses:
Purchased Power and Transmission	136,086	161,480	175,604
Operations and Maintenance	97,031	80,241	87,088
Depreciation	29,971	26,455	23,561
Amortization of Regulatory Assets/(Liabilities), Net	410	4,492	1,892
Amortization of Rate Reduction Bonds	17,632	16,523	15,494
Energy Efficiency Programs	27,802	21,804	16,336
Taxes Other Than Income Taxes	21,458	17,957	16,529
Total Operating Expenses	330,390	328,952	336,504
Operating Income	110,774	88,363	58,657
Interest Expense:
Interest on Long-Term Debt	23,462	20,023	17,988
Interest on Rate Reduction Bonds	1,229	2,335	3,372
Other Interest	1,943	1,254	479
Total Interest Expense	26,634	23,612	21,839
Other Income/(Loss), Net	2,503	1,489	2,597
Income Before Income Tax Expense	86,643	66,240	39,415
Income Tax Expense	32,140	23,186	16,325
Net Income	54,503	43,054	23,090
NSTAR Electric Company [Member]
Operating Revenues	2,300,997	2,403,053	2,366,201
Operating Expenses:
Purchased Power and Transmission	788,252	905,226	1,031,351
Operations and Maintenance	431,802	387,533	351,843
Depreciation	171,070	163,368	158,574
Amortization of Regulatory Assets/(Liabilities), Net	117,682	82,979	19,071
Amortization of Rate Reduction Bonds	90,322	90,322	104,481
Energy Efficiency Programs	201,234	175,747	117,091
Taxes Other Than Income Taxes	119,219	111,705	104,978
Total Operating Expenses	1,919,581	1,916,880	1,887,389
Operating Income	381,416	486,173	478,812
Interest Expense:
Interest on Long-Term Debt	87,100	90,040	90,630
Interest on Rate Reduction Bonds	3,585	7,226	11,235
Other Interest	(20,631)	(27,839)	(30,475)
Total Interest Expense	70,054	69,427	71,390
Other Income/(Loss), Net	2,846	1,434	3,173
Income Before Income Tax Expense	314,208	418,180	410,595
Income Tax Expense	123,966	165,686	162,020
Net Income	$ 190,242	$ 252,494	$ 248,575