INCOME TAXES - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current Income Taxes:
Federal	$ 56,900	$ 106,500	$ 58,900
State	10,500	10,600	31,600
Total Current	67,400	117,100	90,500
Deferred Income Taxes, Net:
Federal	138,400	122,600	433,000
State	71,400	52,200	58,600
Total Deferred	209,812	174,812	491,630
Investment Tax Credits, Net	(3,700)	(2,900)	(3,200)
Income Tax Expense	273,499	288,972	578,892
CL&P
Current Income Taxes:
Federal	68,400	54,200	50,900
State	15,400	20,900	17,400
Total Current	83,800	75,100	68,300
Deferred Income Taxes, Net:
Federal	35,200	48,500	123,900
State	18,800	6,400	(4,600)
Total Deferred	54,005	54,859	119,295
Investment Tax Credits, Net	(800)	(900)	(1,000)
Income Tax Expense	136,971	129,056	186,646
NSTAR Electric
Current Income Taxes:
Federal	82,600	79,300	107,800
State	18,200	30,000	25,600
Total Current	100,800	109,300	133,400
Deferred Income Taxes, Net:
Federal	100	27,900	88,100
State	27,000	13,500	22,400
Total Deferred	27,107	41,438	110,499
Investment Tax Credits, Net	(2,600)	(1,800)	(1,800)
Income Tax Expense	125,313	148,906	242,085
PSNH
Current Income Taxes:
Federal	22,900	12,200	18,600
State	2,200	(500)	6,200
Total Current	25,100	11,700	24,800
Deferred Income Taxes, Net:
Federal	5,800	15,400	52,700
State	10,100	20,500	11,200
Total Deferred	15,917	35,924	63,883
Investment Tax Credits, Net	0	0	0
Income Tax Expense	$ 40,975	$ 47,576	$ 88,675