INCOME TAXES - Reconciliation Between Income Tax Expense and Expected Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 1,190,071	$ 1,329,491	$ 1,574,407
Statutory Federal Income Tax Expense at 21% in 2019 and 2018, and 35% in 2017	249,900	279,200	551,000
Tax Effect of Differences:
Depreciation	1,900	(30,800)	(10,800)
Investment Tax Credit Amortization	(3,700)	(2,900)	(3,200)
State Income Taxes, Net of Federal Impact	24,600	44,400	47,700
Dividends on ESOP	(5,100)	(5,100)	(8,400)
Tax Asset Valuation Allowance/Reserve Adjustments	40,100	5,200	7,000
Excess Stock Benefit	(1,500)	(1,500)	(2,900)
EDIT Amortization	(37,400)	(5,000)	0
Other, Net	4,700	5,500	(1,500)
Income Tax Expense	$ 273,499	$ 288,972	$ 578,892
Effective Tax Rate	23.00%	21.70%	36.80%
CL&P
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 547,823	$ 506,773	$ 563,372
Statutory Federal Income Tax Expense at 21% in 2019 and 2018, and 35% in 2017	115,000	106,400	197,200
Tax Effect of Differences:
Depreciation	(200)	(1,200)	(5,200)
Investment Tax Credit Amortization	(800)	(900)	(1,000)
State Income Taxes, Net of Federal Impact	2,500	14,500	4,500
Tax Asset Valuation Allowance/Reserve Adjustments	24,500	7,100	(9,500)
Excess Stock Benefit	(500)	(100)	(700)
EDIT Amortization	(5,800)	0	0
Other, Net	2,300	3,300	1,300
Income Tax Expense	$ 136,971	$ 129,056	$ 186,646
Effective Tax Rate	25.00%	25.50%	33.10%
NSTAR Electric
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 557,269	$ 531,996	$ 616,811
Statutory Federal Income Tax Expense at 21% in 2019 and 2018, and 35% in 2017	117,000	111,700	215,900
Tax Effect of Differences:
Depreciation	(3,000)	(2,800)	(3,000)
Investment Tax Credit Amortization	(2,600)	(1,800)	(1,800)
State Income Taxes, Net of Federal Impact	35,700	33,200	31,200
Tax Asset Valuation Allowance/Reserve Adjustments	0	1,200	0
Excess Stock Benefit	(500)	(100)	(700)
EDIT Amortization	(22,900)	0	0
Other, Net	1,600	7,500	500
Income Tax Expense	$ 125,313	$ 148,906	$ 242,085
Effective Tax Rate	22.50%	28.00%	39.20%
PSNH
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 175,023	$ 163,452	$ 224,671
Statutory Federal Income Tax Expense at 21% in 2019 and 2018, and 35% in 2017	36,800	34,300	78,600
Tax Effect of Differences:
Depreciation	(800)	100	1,100
Investment Tax Credit Amortization	0	0	0
State Income Taxes, Net of Federal Impact	9,800	15,800	11,300
Tax Asset Valuation Allowance/Reserve Adjustments	0	0	0
Excess Stock Benefit	(200)	(100)	(300)
EDIT Amortization	(4,000)	(4,400)	0
Other, Net	(600)	1,900	(2,000)
Income Tax Expense	$ 40,975	$ 47,576	$ 88,675
Effective Tax Rate	23.40%	29.10%	39.50%