INCOME TAXES - Reconciliation Between Income Tax Expense and Expected Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 1,558,872	$ 1,190,071	$ 1,329,491
Statutory Federal Income Tax Expense at 21%	327,400	249,900	279,200
Tax Effect of Differences:
Depreciation	(11,100)	1,900	(30,800)
Investment Tax Credit Amortization	(3,700)	(3,700)	(2,900)
State Income Taxes,   Net of Federal Impact	44,900	24,600	44,400
Dividends on ESOP	(5,100)	(5,100)	(5,100)
Tax Asset Valuation   Allowance/Reserve Adjustments	33,400	40,100	5,200
Excess Stock Benefit	(6,600)	(1,500)	(1,500)
EDIT Amortization	(48,700)	(37,400)	(5,000)
Other, Net	15,700	4,700	5,500
Income Tax Expense	$ 346,186	$ 273,499	$ 288,972
Effective Tax Rate	22.20%	23.00%	21.70%
The Connecticut Light and Power Company
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 607,636	$ 547,823	$ 506,773
Statutory Federal Income Tax Expense at 21%	127,600	115,000	106,400
Tax Effect of Differences:
Depreciation	400	(200)	(1,200)
Investment Tax Credit Amortization	(700)	(800)	(900)
State Income Taxes,   Net of Federal Impact	(1,200)	2,500	14,500
Tax Asset Valuation   Allowance/Reserve Adjustments	30,700	24,500	7,100
Excess Stock Benefit	(2,300)	(500)	(100)
EDIT Amortization	(9,000)	(5,800)	0
Other, Net	4,200	2,300	3,300
Income Tax Expense	$ 149,702	$ 136,971	$ 129,056
Effective Tax Rate	24.60%	25.00%	25.50%
NSTAR Electric Company
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 575,829	$ 557,269	$ 531,996
Statutory Federal Income Tax Expense at 21%	120,900	117,000	111,700
Tax Effect of Differences:
Depreciation	(3,700)	(3,000)	(2,800)
Investment Tax Credit Amortization	(2,600)	(2,600)	(1,800)
State Income Taxes,   Net of Federal Impact	36,000	35,700	33,200
Tax Asset Valuation   Allowance/Reserve Adjustments	0	0	1,200
Excess Stock Benefit	(2,300)	(500)	(100)
EDIT Amortization	(20,400)	(22,900)	0
Other, Net	2,900	1,600	7,500
Income Tax Expense	$ 130,828	$ 125,313	$ 148,906
Effective Tax Rate	22.70%	22.50%	28.00%
Public Service Company of New Hampshire
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 178,992	$ 175,023	$ 163,452
Statutory Federal Income Tax Expense at 21%	37,600	36,800	34,300
Tax Effect of Differences:
Depreciation	(1,400)	(800)	100
Investment Tax Credit Amortization	0	0	0
State Income Taxes,   Net of Federal Impact	9,800	9,800	15,800
Tax Asset Valuation   Allowance/Reserve Adjustments	0	0	0
Excess Stock Benefit	(800)	(200)	(100)
EDIT Amortization	(15,400)	(4,000)	(4,400)
Other, Net	1,900	(600)	1,900
Income Tax Expense	$ 31,680	$ 40,975	$ 47,576
Effective Tax Rate	17.70%	23.40%	29.10%