ASSETS HELD FOR SALE - Narrative (Details) $ in Thousands				12 Months Ended
	Jan. 30, 2018 USD ($)	Jan. 10, 2018 USD ($)	Oct. 11, 2017 USD ($) agreement	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Regulatory Assets [Line Items]
Regulatory assets				$ 5,239,300	$ 4,526,300
Income before income tax expense				1,574,407	1,504,818	$ 1,425,971
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Purchase price in purchase sale agreement for thermal generation assets			$ 175,000
Regulatory assets				940,800	362,700
Difference between book value of hydroelectric generation assets and expected proceeds from sale		$ 25,000
Income before income tax expense				224,671	214,349	187,502
Liabilities held for sale				1,200
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Regulatory assets				1,645,200	1,727,100
Income before income tax expense				563,372	542,562	476,756
Subsequent Event
Regulatory Assets [Line Items]
Number of purchase sale agreements | agreement			2
Purchase price in purchase sale agreement for thermal generation assets			$ 175,000
Purchase price in purchase sale agreement for hydroelectric assets			$ 83,000
Subsequent Event | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Purchase price is sale agreement adjusted for other expenses		40,900
Proceeds from assets held for sale		$ 134,100
Rate reduction bonds issued	$ 690,000
Period of appeal for rate reduction bonds	30 days
Deferred Costs from Generation Asset Sale [Member]
Regulatory Assets [Line Items]
Regulatory assets				516,100	0
Deferred Costs from Generation Asset Sale [Member] | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets				516,100	0
Deferred Costs from Generation Asset Sale [Member] | The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Regulatory assets				0	0
Domestic Tax Authority | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Income before income tax expense				$ 60,000	$ 65,300	$ 56,900