CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Revenues	$ 7,639,129	$ 7,954,827	$ 7,741,856
Operating Expenses:
Purchased Power, Fuel and Transmission	2,500,828	3,086,905	3,021,550
Operations and Maintenance	1,323,549	1,329,289	1,427,589
Depreciation	715,466	665,856	614,657
Amortization of Regulatory Assets, Net	71,696	22,339	10,704
Energy Efficiency Programs	533,659	495,701	473,127
Taxes Other Than Income Taxes	634,072	590,573	561,380
Total Operating Expenses	5,779,270	6,190,663	6,109,007
Operating Income	1,859,859	1,764,164	1,632,849
Interest Expense	400,961	372,420	362,106
Other Income, Net	45,920	34,227	24,619
Income Before Income Tax Expense	1,504,818	1,425,971	1,295,362
Income Tax Expense	554,997	539,967	468,297
Net Income	949,821	886,004	827,065
Net Income Attributable to Noncontrolling Interests	7,519	7,519	7,519
Net Income Attributable to Common Shareholders	$ 942,302	$ 878,485	$ 819,546
Basic Earnings Per Common Share (in dollars per share)	$ 2.97	$ 2.77	$ 2.59
Diluted Earnings Per Common Share (in dollars per share)	$ 2.96	$ 2.76	$ 2.58
Weighted Average Common Shares Outstanding:
Basic (in shares)	317,650,180	317,336,881	316,136,748
Diluted (in shares)	318,454,239	318,432,687	317,417,414
The Connecticut Light And Power Company [Member]
Operating Revenues	$ 2,805,955	$ 2,802,675	$ 2,692,582
Operating Expenses:
Purchased Power, Fuel and Transmission	919,723	1,054,313	982,876
Operations and Maintenance	490,069	487,281	494,578
Depreciation	230,489	215,289	188,837
Amortization of Regulatory Assets, Net	38,765	12,318	59,336
Energy Efficiency Programs	154,015	153,725	156,335
Taxes Other Than Income Taxes	299,719	268,688	255,370
Total Operating Expenses	2,132,780	2,191,614	2,137,332
Operating Income	673,175	611,061	555,250
Interest Expense	144,110	145,795	147,421
Other Income, Net	13,497	11,490	13,376
Income Before Income Tax Expense	542,562	476,756	421,205
Income Tax Expense	208,308	177,396	133,451
Net Income	334,254	299,360	287,754
NSTAR Electric Company [Member]
Operating Revenues	2,557,878	2,681,342	2,536,677
Operating Expenses:
Purchased Power, Fuel and Transmission	953,356	1,190,191	1,122,298
Operations and Maintenance	393,170	306,528	326,972
Depreciation	213,115	196,770	188,693
Amortization of Regulatory Assets/(Liabilities), Net	30,083	(12,989)	(6,330)
Energy Efficiency Programs	277,608	224,755	193,516
Taxes Other Than Income Taxes	136,759	133,260	133,072
Total Operating Expenses	2,004,091	2,038,515	1,958,221
Operating Income	553,787	642,827	578,456
Interest Expense	84,005	75,347	77,878
Other Income, Net	10,690	5,106	4,491
Income Before Income Tax Expense	480,472	572,586	505,069
Income Tax Expense	187,767	228,044	201,981
Net Income	292,705	344,542	303,088
Public Service Company Of New Hampshire [Member]
Operating Revenues	959,482	972,203	959,500
Operating Expenses:
Purchased Power, Fuel and Transmission	210,786	247,721	313,732
Operations and Maintenance	260,779	276,554	261,848
Depreciation	116,519	105,372	98,436
Amortization of Regulatory Assets/(Liabilities), Net	11,170	16,276	(29,602)
Energy Efficiency Programs	14,204	14,324	14,286
Taxes Other Than Income Taxes	82,964	81,779	71,417
Total Operating Expenses	696,422	742,026	730,117
Operating Income	263,060	230,177	229,383
Interest Expense	50,040	45,990	45,349
Other Income, Net	1,329	3,315	2,045
Income Before Income Tax Expense	214,349	187,502	186,079
Income Tax Expense	82,364	73,060	72,135
Net Income	131,985	114,442	113,944
Western Massachusetts Electric Company [Member]
Operating Revenues	484,192	518,128	493,423
Operating Expenses:
Purchased Power, Fuel and Transmission	131,449	177,172	172,876
Operations and Maintenance	96,712	86,360	89,406
Depreciation	46,147	43,362	41,886
Amortization of Regulatory Assets/(Liabilities), Net	4,249	14,545	(6,228)
Energy Efficiency Programs	44,179	42,867	42,937
Taxes Other Than Income Taxes	41,079	38,302	34,907
Total Operating Expenses	363,815	402,608	375,784
Operating Income	120,377	115,520	117,639
Interest Expense	24,425	24,792	24,931
Other Income, Net	142	2,748	2,379
Income Before Income Tax Expense	96,094	93,476	95,087
Income Tax Expense	38,022	36,970	37,268
Net Income	$ 58,072	$ 56,506	$ 57,819