INCOME TAXES - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current Income Taxes:
Federal	$ 38,900	$ 6,200	$ 4,400
State	53,000	45,700	24,500
Total Current	91,900	51,900	28,900
Deferred Income Taxes, Net:
Federal	427,900	436,100	406,800
State	38,600	55,600	36,500
Total Deferred	466,463	491,736	443,259
Investment Tax Credits, Net	(3,400)	(3,600)	(3,900)
Income Tax Expense	554,997	539,967	468,297
The Connecticut Light And Power Company [Member]
Current Income Taxes:
Federal	27,300	26,900	(200)
State	13,300	15,800	4,300
Total Current	40,600	42,700	4,100
Deferred Income Taxes, Net:
Federal	157,600	135,800	138,000
State	11,300	200	(7,100)
Total Deferred	168,919	135,994	130,949
Investment Tax Credits, Net	(1,200)	(1,300)	(1,500)
Income Tax Expense	208,308	177,396	133,451
NSTAR Electric Company [Member]
Current Income Taxes:
Federal	73,900	36,300	75,000
State	35,000	19,800	20,200
Total Current	108,900	56,100	95,200
Deferred Income Taxes, Net:
Federal	78,300	147,500	88,000
State	1,900	25,700	20,100
Total Deferred	80,200	173,155	108,133
Investment Tax Credits, Net	(1,300)	(1,300)	(1,300)
Income Tax Expense	187,767	228,044	201,981
Public Service Company Of New Hampshire [Member]
Current Income Taxes:
Federal	(13,700)	(16,700)	(22,600)
State	8,800	6,000	(100)
Total Current	(4,900)	(10,700)	(22,700)
Deferred Income Taxes, Net:
Federal	79,500	74,500	79,600
State	7,800	9,300	15,200
Total Deferred	87,345	83,776	94,813
Investment Tax Credits, Net	0	0	0
Income Tax Expense	82,364	73,060	72,135
Western Massachusetts Electric Company [Member]
Current Income Taxes:
Federal	12,500	(3,500)	1,900
State	4,500	1,600	1,800
Total Current	17,000	(1,900)	3,700
Deferred Income Taxes, Net:
Federal	18,300	33,400	28,100
State	3,200	6,000	6,000
Total Deferred	21,498	39,428	34,108
Investment Tax Credits, Net	(500)	(500)	(500)
Income Tax Expense	$ 38,022	$ 36,970	$ 37,268