CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Revenues	$ 7,741,856	$ 7,301,204	$ 6,273,787
Operating Expenses:
Purchased Power and Transmission	3,021,550	2,482,954	2,084,364
Operations and Maintenance	1,427,589	1,514,986	1,583,070
Depreciation	614,657	610,777	519,010
Amortization of Regulatory Assets/(Liabilities), Net	10,704	206,322	79,762
Amortization of Rate Reduction Bonds	0	42,581	142,019
Energy Efficiency Programs	473,127	401,919	313,149
Taxes Other Than Income Taxes	561,380	512,230	434,207
Total Operating Expenses	6,109,007	5,771,769	5,155,581
Operating Income	1,632,849	1,529,435	1,118,206
Interest Expense:
Interest on Long-Term Debt	345,001	340,970	316,987
Interest on Rate Reduction Bonds	0	422	6,168
Other Interest	17,105	(2,693)	6,790
Total Interest Expense	362,106	338,699	329,945
Other Income/(Loss), Net	24,619	29,894	19,742
Income Before Income Tax Expense	1,295,362	1,220,630	808,003
Income Tax Expense	468,297	426,941	274,926
Net Income	827,065	793,689	533,077
Net Income Attributable to Noncontrolling Interests	7,519	7,682	7,132
Net Income Attributable to Controlling Interests	819,546	786,007	525,945
Basic Earnings Per Common Share	$ 2.59	$ 2.49	$ 1.90
Diluted Earnings Per Common Share	$ 2.58	$ 2.49	$ 1.89
Weighted Average Common Shares Outstanding:
Basic	316,136,748	315,311,387	277,209,819
Diluted	317,417,414	316,211,160	277,993,631
The Connecticut Light And Power Company [Member]
Operating Revenues	2,692,582	2,442,341	2,407,449
Operating Expenses:
Purchased Power and Transmission	982,876	872,769	858,231
Operations and Maintenance	494,578	523,247	635,733
Depreciation	188,837	177,603	166,853
Amortization of Regulatory Assets/(Liabilities), Net	59,336	4,870	14,372
Energy Efficiency Programs	156,335	89,858	89,299
Taxes Other Than Income Taxes	255,370	234,418	215,972
Total Operating Expenses	2,137,332	1,902,765	1,980,460
Operating Income	555,250	539,576	426,989
Interest Expense:
Interest on Long-Term Debt	135,656	130,620	124,894
Other Interest	11,765	3,030	8,233
Total Interest Expense	147,421	133,650	133,127
Other Income/(Loss), Net	13,376	15,149	10,300
Income Before Income Tax Expense	421,205	421,075	304,162
Income Tax Expense	133,451	141,663	94,437
Net Income	287,754	279,412	209,725
NSTAR Electric Company [Member]
Operating Revenues	2,536,677	2,493,479	2,300,997
Operating Expenses:
Purchased Power and Transmission	1,122,298	849,149	788,252
Operations and Maintenance	326,972	376,360	431,802
Depreciation	188,693	180,298	171,070
Amortization of Regulatory Assets/(Liabilities), Net	(6,330)	230,148	117,682
Amortization of Rate Reduction Bonds	0	15,054	90,322
Energy Efficiency Programs	193,516	206,536	201,234
Taxes Other Than Income Taxes	133,072	127,778	119,219
Total Operating Expenses	1,958,221	1,985,323	1,919,581
Operating Income	578,456	508,156	381,416
Interest Expense:
Interest on Long-Term Debt	77,140	79,088	87,100
Interest on Rate Reduction Bonds	0	399	3,585
Other Interest	738	(9,104)	(20,631)
Total Interest Expense	77,878	70,383	70,054
Other Income/(Loss), Net	4,491	3,639	2,846
Income Before Income Tax Expense	505,069	441,412	314,208
Income Tax Expense	201,981	172,866	123,966
Net Income	303,088	268,546	190,242
Public Service Company Of New Hampshire [Member]
Operating Revenues	959,500	935,402	988,013
Operating Expenses:
Purchased Power and Transmission	313,732	269,754	319,253
Operations and Maintenance	261,848	267,797	263,234
Depreciation	98,436	91,581	87,602
Amortization of Regulatory Assets/(Liabilities), Net	(29,602)	(20,387)	(24,086)
Amortization of Rate Reduction Bonds	0	19,748	56,645
Energy Efficiency Programs	14,286	14,494	14,245
Taxes Other Than Income Taxes	71,417	67,196	66,025
Total Operating Expenses	730,117	710,183	782,918
Operating Income	229,383	225,219	205,095
Interest Expense:
Interest on Long-Term Debt	45,116	44,370	46,228
Interest on Rate Reduction Bonds	0	(154)	2,687
Other Interest	233	1,960	1,313
Total Interest Expense	45,349	46,176	50,228
Other Income/(Loss), Net	2,045	3,455	3,008
Income Before Income Tax Expense	186,079	182,498	157,875
Income Tax Expense	72,135	71,101	60,993
Net Income	113,944	111,397	96,882
Western Massachusetts Electric Company [Member]
Operating Revenues	493,423	472,724	441,164
Operating Expenses:
Purchased Power and Transmission	172,876	147,059	136,086
Operations and Maintenance	89,406	96,194	97,031
Depreciation	41,886	37,568	29,971
Amortization of Regulatory Assets/(Liabilities), Net	(6,228)	(3,206)	410
Amortization of Rate Reduction Bonds	0	7,780	17,632
Energy Efficiency Programs	42,937	39,524	27,802
Taxes Other Than Income Taxes	34,907	28,458	21,458
Total Operating Expenses	375,784	353,377	330,390
Operating Income	117,639	119,347	110,774
Interest Expense:
Interest on Long-Term Debt	24,245	23,625	23,462
Interest on Rate Reduction Bonds	0	177	1,229
Other Interest	686	1,049	1,943
Total Interest Expense	24,931	24,851	26,634
Other Income/(Loss), Net	2,379	3,310	2,503
Income Before Income Tax Expense	95,087	97,806	86,643
Income Tax Expense	37,268	37,368	32,140
Net Income	$ 57,819	$ 60,438	$ 54,503