INCOME TAXES - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current Income Taxes:
Federal	$ 73,600	$ 56,900	$ 106,500
State	19,100	10,500	10,600
Total Current	92,700	67,400	117,100
Deferred Income Taxes, Net:
Federal	173,500	138,400	122,600
State	83,700	71,400	52,200
Total Deferred	257,154	209,812	174,812
Investment Tax Credits, Net	(3,700)	(3,700)	(2,900)
Income Tax Expense	346,186	273,499	288,972
The Connecticut Light and Power Company
Current Income Taxes:
Federal	12,000	68,400	54,200
State	(6,100)	15,400	20,900
Total Current	5,900	83,800	75,100
Deferred Income Taxes, Net:
Federal	101,100	35,200	48,500
State	43,400	18,800	6,400
Total Deferred	144,527	54,005	54,859
Investment Tax Credits, Net	(700)	(800)	(900)
Income Tax Expense	149,702	136,971	129,056
NSTAR Electric Company
Current Income Taxes:
Federal	53,900	82,600	79,300
State	6,900	18,200	30,000
Total Current	60,800	100,800	109,300
Deferred Income Taxes, Net:
Federal	33,800	100	27,900
State	38,800	27,000	13,500
Total Deferred	72,595	27,107	41,438
Investment Tax Credits, Net	(2,600)	(2,600)	(1,800)
Income Tax Expense	130,828	125,313	148,906
Public Service Company of New Hampshire
Current Income Taxes:
Federal	20,600	22,900	12,200
State	3,800	2,200	(500)
Total Current	24,400	25,100	11,700
Deferred Income Taxes, Net:
Federal	(1,300)	5,800	15,400
State	8,600	10,100	20,500
Total Deferred	7,337	15,917	35,924
Investment Tax Credits, Net	0	0	0
Income Tax Expense	$ 31,680	$ 40,975	$ 47,576