INCOME TAXES - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current Income Taxes:
Federal	$ 58,900	$ 38,900	$ 6,200
State	31,600	53,000	45,700
Total Current	90,500	91,900	51,900
Deferred Income Taxes, Net:
Federal	433,000	427,900	436,100
State	58,600	38,600	55,600
Total Deferred	491,630	466,463	491,736
Investment Tax Credits, Net	(3,200)	(3,400)	(3,600)
Income Tax Expense	578,892	554,997	539,967
The Connecticut Light And Power Company
Current Income Taxes:
Federal	50,900	27,300	26,900
State	17,400	13,300	15,800
Total Current	68,300	40,600	42,700
Deferred Income Taxes, Net:
Federal	123,900	157,600	135,800
State	(4,600)	11,300	200
Total Deferred	119,295	168,919	135,994
Investment Tax Credits, Net	(1,000)	(1,200)	(1,300)
Income Tax Expense	186,646	208,308	177,396
NSTAR Electric Company
Current Income Taxes:
Federal	107,800	86,400	32,800
State	25,600	39,500	21,400
Total Current	133,400	125,900	54,200
Deferred Income Taxes, Net:
Federal	88,100	96,600	180,900
State	22,400	5,100	31,700
Total Deferred	110,499	101,698	212,583
Investment Tax Credits, Net	(1,800)	(1,800)	(1,800)
Income Tax Expense	242,085	225,789	265,014
Public Service Company Of New Hampshire
Current Income Taxes:
Federal	18,600	(13,700)	(16,700)
State	6,200	8,800	6,000
Total Current	24,800	(4,900)	(10,700)
Deferred Income Taxes, Net:
Federal	52,700	79,500	74,500
State	11,200	7,800	9,300
Total Deferred	63,883	87,345	83,776
Investment Tax Credits, Net	0	0	0
Income Tax Expense	$ 88,675	$ 82,364	$ 73,060