INCOME TAXES - Reconciliation Between Income Tax Expense and Expected Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 1,504,818	$ 1,425,971	$ 1,295,362
Federal tax expense (in percentage)	35.00%	35.00%	35.00%
Statutory Federal Income Tax Expense at 35%	$ 526,700	$ 499,100	$ 453,400
Tax Effect of Differences:
Depreciation	(3,400)	(4,600)	(5,600)
Investment Tax Credit Amortization	(3,400)	(3,600)	(3,900)
Other Federal Tax Credits	(3,500)	(3,800)	(3,500)
State Income Taxes, Net of Federal Impact	56,200	61,100	42,500
Dividends on ESOP	(8,400)	(8,100)	(8,000)
Tax Asset Valuation Allowance/Reserve Adjustments	3,300	4,700	(2,900)
Excess Stock Benefit (1)	(19,100)	0	0
Other, Net	6,600	(4,800)	(3,700)
Income Tax Expense	$ 554,997	$ 539,967	$ 468,297
Effective Income Tax Rate Reconciliation, Percent	36.90%	37.90%	36.20%
The Connecticut Light And Power Company [Member]
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 542,562	$ 476,756	$ 421,205
Statutory Federal Income Tax Expense at 35%	189,900	166,900	147,400
Tax Effect of Differences:
Depreciation	1,600	(1,700)	(3,600)
Investment Tax Credit Amortization	(1,200)	(1,300)	(1,500)
Other Federal Tax Credits	0	0	0
State Income Taxes, Net of Federal Impact	14,500	9,200	4,400
Tax Asset Valuation Allowance/Reserve Adjustments	1,500	1,200	(6,300)
Excess Stock Benefit (1)	(900)	0	0
Other, Net	2,900	3,100	(6,900)
Income Tax Expense	$ 208,308	$ 177,396	$ 133,451
Effective Income Tax Rate Reconciliation, Percent	38.40%	37.20%	31.70%
NSTAR Electric Company [Member]
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 480,472	$ 572,586	$ 505,069
Statutory Federal Income Tax Expense at 35%	168,200	200,400	176,800
Tax Effect of Differences:
Depreciation	(3,400)	(1,400)	(1,300)
Investment Tax Credit Amortization	(1,300)	(1,300)	(1,300)
Other Federal Tax Credits	0	0	0
State Income Taxes, Net of Federal Impact	24,000	29,600	26,200
Tax Asset Valuation Allowance/Reserve Adjustments	0	0	0
Excess Stock Benefit (1)	(1,000)	0	0
Other, Net	1,300	700	1,600
Income Tax Expense	$ 187,767	$ 228,044	$ 201,981
Effective Income Tax Rate Reconciliation, Percent	39.10%	39.80%	40.00%
Public Service Company Of New Hampshire [Member]
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 214,349	$ 187,502	$ 186,079
Statutory Federal Income Tax Expense at 35%	75,000	65,600	65,100
Tax Effect of Differences:
Depreciation	1,000	500	300
Investment Tax Credit Amortization	0	0	0
Other Federal Tax Credits	(3,500)	(3,800)	(3,500)
State Income Taxes, Net of Federal Impact	10,800	9,900	9,800
Tax Asset Valuation Allowance/Reserve Adjustments	0	0	0
Excess Stock Benefit (1)	(400)	0	0
Other, Net	(500)	900	400
Income Tax Expense	$ 82,364	$ 73,060	$ 72,135
Effective Income Tax Rate Reconciliation, Percent	38.40%	39.00%	38.70%
Western Massachusetts Electric Company [Member]
Income Tax Contingency [Line Items]
Income Before Income Tax Expense	$ 96,094	$ 93,476	$ 95,087
Statutory Federal Income Tax Expense at 35%	33,600	32,700	33,300
Tax Effect of Differences:
Depreciation	300	(300)	(200)
Investment Tax Credit Amortization	(500)	(500)	(500)
Other Federal Tax Credits	0	0	0
State Income Taxes, Net of Federal Impact	5,000	4,900	5,000
Tax Asset Valuation Allowance/Reserve Adjustments	0	0	0
Excess Stock Benefit (1)	(200)	0	0
Other, Net	(200)	200	(300)
Income Tax Expense	$ 38,022	$ 36,970	$ 37,268
Effective Income Tax Rate Reconciliation, Percent	39.60%	39.60%	39.20%