ASSETS HELD FOR SALE - Narrative (Details) $ in Thousands			1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 16, 2018 USD ($) license	Jan. 10, 2018 USD ($)	Oct. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	May 08, 2018 USD ($)	Dec. 31, 2017 USD ($)
Regulatory Assets [Line Items]
Proceeds from sale of property						$ 193,924	$ 0
Regulatory assets held for sale				$ 5,153,300		5,153,300			$ 5,239,300
Pre-tax income associated with assets held for sale				357,538	$ 415,068	1,027,829	1,204,137
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Thermal generation assets			$ 175,000	17,300
Purchase price in purchase sale agreement		$ 40,900
Proceeds from purchase and sale agreement	$ 77,300	$ 134,100
Proceeds from sale of property				116,800		193,924	0
Number of licenses | license	6
Hydroelectric assets	$ 5,700		$ 83,000
Regulatory assets held for sale				953,500		953,500			940,800
Difference between book value of hydroelectric generation assets and expected proceeds from sale						17,200
Pre-tax income associated with assets held for sale				55,975	55,735	139,394	164,754
Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Regulatory assets held for sale				0		0			516,100
Deferred Costs from Generation Asset Sale | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets held for sale				0		0			$ 516,100
Domestic Tax Authority | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Pre-tax income associated with assets held for sale				$ 700	$ 14,600	$ 9,900	$ 44,700
Rate Reduction Bonds | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Amount of securitized rate reduction bonds issued								$ 635,700