INCOME TAXES - Reconciliation Between Income Tax Expense and Expected Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 1,574,407	$ 1,504,818	$ 1,425,971
Federal tax expense (in percentage)	35.00%	35.00%	35.00%
Statutory Federal Income Tax Expense at 35%	$ 551,000	$ 526,700	$ 499,100
Tax Effect of Differences:
Depreciation	(10,800)	(3,400)	(4,600)
Investment Tax Credit Amortization	(3,200)	(3,400)	(3,600)
Other Federal Tax Credits	0	(3,500)	(3,800)
State Income Taxes, Net of Federal Impact	47,700	56,200	61,100
Dividends on ESOP	(8,400)	(8,400)	(8,100)
Tax Asset Valuation Allowance/Reserve Adjustments	7,000	3,300	4,700
Excess Stock Benefit	(2,900)	(19,100)	0
Other, Net	(1,500)	6,600	(4,800)
Income Tax Expense	$ 578,892	$ 554,997	$ 539,967
Effective Tax Rate	36.80%	36.90%	37.90%
The Connecticut Light And Power Company
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 563,372	$ 542,562	$ 476,756
Statutory Federal Income Tax Expense at 35%	197,200	189,900	166,900
Tax Effect of Differences:
Depreciation	(5,200)	1,600	(1,700)
Investment Tax Credit Amortization	(1,000)	(1,200)	(1,300)
Other Federal Tax Credits	0	0	0
State Income Taxes, Net of Federal Impact	4,500	14,500	9,200
Tax Asset Valuation Allowance/Reserve Adjustments	(9,500)	1,500	1,200
Excess Stock Benefit	(700)	(900)	0
Other, Net	1,300	2,900	3,100
Income Tax Expense	$ 186,646	$ 208,308	$ 177,396
Effective Tax Rate	33.10%	38.40%	37.20%
NSTAR Electric Company
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 616,811	$ 576,566	$ 666,062
Statutory Federal Income Tax Expense at 35%	215,900	201,800	233,100
Tax Effect of Differences:
Depreciation	(3,000)	(3,100)	(1,700)
Investment Tax Credit Amortization	(1,800)	(1,800)	(1,800)
Other Federal Tax Credits	0	0	0
State Income Taxes, Net of Federal Impact	31,200	29,000	34,500
Tax Asset Valuation Allowance/Reserve Adjustments	0	0	0
Excess Stock Benefit	(700)	(1,200)	0
Other, Net	500	1,100	900
Income Tax Expense	$ 242,085	$ 225,789	$ 265,014
Effective Tax Rate	39.20%	39.20%	39.80%
Public Service Company Of New Hampshire
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 224,671	$ 214,349	$ 187,502
Statutory Federal Income Tax Expense at 35%	78,600	75,000	65,600
Tax Effect of Differences:
Depreciation	1,100	1,000	500
Investment Tax Credit Amortization	0	0	0
Other Federal Tax Credits	0	(3,500)	(3,800)
State Income Taxes, Net of Federal Impact	11,300	10,800	9,900
Tax Asset Valuation Allowance/Reserve Adjustments	0	0	0
Excess Stock Benefit	(300)	(400)	0
Other, Net	(2,000)	(500)	900
Income Tax Expense	$ 88,675	$ 82,364	$ 73,060
Effective Tax Rate	39.50%	38.40%	39.00%