INCOME TAXES - Reconciliation Between Income Tax Expense and Expected Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 1,329,491	$ 1,574,407	$ 1,504,818
Federal tax expense (in percentage)	21.00%	35.00%	35.00%
Statutory Federal Income Tax Expense at 21% in 2018 and 35% in 2017 and 2016	$ 279,200	$ 551,000	$ 526,700
Tax Effect of Differences:
Depreciation	(30,800)	(10,800)	(3,400)
Investment Tax Credit Amortization	(2,900)	(3,200)	(3,400)
Other Federal Tax Credits	0	0	(3,500)
State Income Taxes, Net of Federal Impact	44,400	47,700	56,200
Dividends on ESOP	(5,100)	(8,400)	(8,400)
Tax Asset Valuation Allowance/Reserve Adjustments	5,200	7,000	3,300
Excess Stock Benefit	(1,500)	(2,900)	(19,100)
Other, Net	500	(1,500)	6,600
Income Tax Expense	$ 288,972	$ 578,892	$ 554,997
Effective Tax Rate	21.70%	36.80%	36.90%
The Connecticut Light And Power Company
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 506,773	$ 563,372	$ 542,562
Statutory Federal Income Tax Expense at 21% in 2018 and 35% in 2017 and 2016	106,400	197,200	189,900
Tax Effect of Differences:
Depreciation	(1,200)	(5,200)	1,600
Investment Tax Credit Amortization	(900)	(1,000)	(1,200)
Other Federal Tax Credits	0	0	0
State Income Taxes, Net of Federal Impact	14,500	4,500	14,500
Tax Asset Valuation Allowance/Reserve Adjustments	7,100	(9,500)	1,500
Excess Stock Benefit	(100)	(700)	(900)
Other, Net	3,300	1,300	2,900
Income Tax Expense	$ 129,056	$ 186,646	$ 208,308
Effective Tax Rate	25.50%	33.10%	38.40%
NSTAR Electric Company
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 531,996	$ 616,811	$ 576,566
Statutory Federal Income Tax Expense at 21% in 2018 and 35% in 2017 and 2016	111,700	215,900	201,800
Tax Effect of Differences:
Depreciation	(2,800)	(3,000)	(3,100)
Investment Tax Credit Amortization	(1,800)	(1,800)	(1,800)
Other Federal Tax Credits	0	0	0
State Income Taxes, Net of Federal Impact	33,200	31,200	29,000
Tax Asset Valuation Allowance/Reserve Adjustments	1,200	0	0
Excess Stock Benefit	(100)	(700)	(1,200)
Other, Net	7,500	500	1,100
Income Tax Expense	$ 148,906	$ 242,085	$ 225,789
Effective Tax Rate	28.00%	39.20%	39.20%
Public Service Company Of New Hampshire
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 163,452	$ 224,671	$ 214,349
Statutory Federal Income Tax Expense at 21% in 2018 and 35% in 2017 and 2016	34,300	78,600	75,000
Tax Effect of Differences:
Depreciation	100	1,100	1,000
Investment Tax Credit Amortization	0	0	0
Other Federal Tax Credits	0	0	(3,500)
State Income Taxes, Net of Federal Impact	15,800	11,300	10,800
Tax Asset Valuation Allowance/Reserve Adjustments	0	0	0
Excess Stock Benefit	(100)	(300)	(400)
Other, Net	(2,500)	(2,000)	(500)
Income Tax Expense	$ 47,576	$ 88,675	$ 82,364
Effective Tax Rate	29.10%	39.50%	38.40%