INCOME TAXES - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current Income Taxes:
Federal	$ 106,500	$ 58,900	$ 38,900
State	10,600	31,600	53,000
Total Current	117,100	90,500	91,900
Deferred Income Taxes, Net:
Federal	122,600	433,000	427,900
State	52,200	58,600	38,600
Total Deferred	174,812	491,630	466,463
Investment Tax Credits, Net	(2,900)	(3,200)	(3,400)
Income Tax Expense	288,972	578,892	554,997
The Connecticut Light And Power Company
Current Income Taxes:
Federal	54,200	50,900	27,300
State	20,900	17,400	13,300
Total Current	75,100	68,300	40,600
Deferred Income Taxes, Net:
Federal	48,500	123,900	157,600
State	6,400	(4,600)	11,300
Total Deferred	54,859	119,295	168,919
Investment Tax Credits, Net	(900)	(1,000)	(1,200)
Income Tax Expense	129,056	186,646	208,308
NSTAR Electric Company
Current Income Taxes:
Federal	79,300	107,800	86,400
State	30,000	25,600	39,500
Total Current	109,300	133,400	125,900
Deferred Income Taxes, Net:
Federal	27,900	88,100	96,600
State	13,500	22,400	5,100
Total Deferred	41,438	110,499	101,698
Investment Tax Credits, Net	(1,800)	(1,800)	(1,800)
Income Tax Expense	148,906	242,085	225,789
Public Service Company Of New Hampshire
Current Income Taxes:
Federal	12,200	18,600	(13,700)
State	(500)	6,200	8,800
Total Current	11,700	24,800	(4,900)
Deferred Income Taxes, Net:
Federal	15,400	52,700	79,500
State	20,500	11,200	7,800
Total Deferred	35,924	63,883	87,345
Investment Tax Credits, Net	0	0	0
Income Tax Expense	$ 47,576	$ 88,675	$ 82,364