INCOME TAXES - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Proceeds from depreciation deduction due to income tax refunds	$ 275,000,000
Decrease in expected tax payments	300,000,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Valuation allowance for tax credit and operating loss carryforward	4,500,000	$ 3,100,000
Minimum [Member]
Operating Loss Carryforwards [Line Items]
Decrease in unrecognized tax benefits	0
Maximum [Member]
Operating Loss Carryforwards [Line Items]
Decrease in unrecognized tax benefits	1,600,000
Eversource [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	1,300,000	(1,300,000)
The Connecticut Light And Power Company [Member]
Operating Loss Carryforwards [Line Items]
Proceeds from depreciation deduction due to income tax refunds	105,000,000
The Connecticut Light And Power Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	1,300,000	$ (900,000)
NSTAR Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Proceeds from depreciation deduction due to income tax refunds	72,000,000
Public Service Company Of New Hampshire [Member]
Operating Loss Carryforwards [Line Items]
Proceeds from depreciation deduction due to income tax refunds	46,000,000
Western Massachusetts Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Proceeds from depreciation deduction due to income tax refunds	$ 25,000,000
Tax Year 2016 and 2017 [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Increase in state corporate tax rate (in percentage)	9.00%
Tax Year 2018 [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Increase in state corporate tax rate (in percentage)	8.25%