GENERATION ASSET SALE - Narrative (Details) $ in Thousands				1 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended
	Jul. 16, 2018 contract	Jan. 10, 2018 USD ($)	Oct. 11, 2017 USD ($)	Oct. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Regulatory Assets [Line Items]
Purchase price in purchase sale agreement for thermal generation assets			$ 175,000
Purchase price in purchase sale agreement for hydroelectric assets			$ 83,000
Proceeds from the Sale of PSNH Generation Assets								$ 193,924	$ 0	$ 0
Number of purchase sale agreements | contract	6
Total Regulatory Assets								5,145,900	5,239,300
Pre-tax incmoe associated with generation assets								1,329,491	1,574,407	1,504,818
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Purchase price in purchase sale agreement for thermal generation assets		$ 175,000				$ 17,300
Purchase price in purchase sale agreement for hydroelectric assets				$ 83,000	$ 5,800
Purchase price is sale agreement adjusted for other expenses		$ 40,900
Proceeds from the Sale of PSNH Generation Assets						$ 116,800		193,924	0	0
Proceeds from assets held for sale					$ 77,200
Difference between book value of hydroelectric generation assets and expected proceeds from sale							$ 17,300
Total Regulatory Assets								929,500	940,800
Pre-tax incmoe associated with generation assets								$ 163,452	224,671	214,349
Domestic Tax Authority | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Pre-tax incmoe associated with generation assets							$ 9,900		$ 60,000	$ 65,300