ASSETS HELD FOR SALE - Narrative (Details) - USD ($) $ in Thousands				3 Months Ended
	Jan. 30, 2018	Jan. 10, 2018	Oct. 11, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets held for sale				$ 5,338,700		$ 5,239,300
Pre-tax income associated with assets held for sale				355,192	$ 419,167
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Thermal generation assets			$ 175,000
Hydroelectric assets			$ 83,000
Purchase price in purchase sale agreement		$ 40,900
Proceeds from purchase and sale agreement		$ 134,100
Regulatory assets held for sale				938,700		940,800
Difference between book value of hydroelectric generation assets and expected proceeds from sale				23,000
Rate reduction bonds issued	$ 690,000
Pre-tax income associated with assets held for sale				47,744	56,642
Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Regulatory assets held for sale				539,600		516,100
Deferred Costs from Generation Asset Sale | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets held for sale				539,600		$ 516,100
Domestic Tax Authority | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Pre-tax income associated with assets held for sale				$ 5,900	$ 14,800