ASSETS HELD FOR SALE - Narrative (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended			6 Months Ended
	Jan. 10, 2018	Oct. 31, 2017	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	May 08, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Proceeds from sale of property						$ 116,809	$ 0
Regulatory assets held for sale			$ 5,396,300			5,396,300			$ 5,239,300
Pre-tax income associated with assets held for sale			315,099		$ 369,901	670,291	789,069
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Thermal generation assets		$ 175,000	17,000
Hydroelectric assets		$ 83,000
Purchase price in purchase sale agreement	$ 40,900
Proceeds from purchase and sale agreement	$ 134,100
Purchase and sale agreement, amount in escrow			3,500
Proceeds from sale of property			116,800			116,809	0
Regulatory assets held for sale			989,200			989,200			940,800
Difference between book value of hydroelectric generation assets and expected proceeds from sale				$ 23,200
Pre-tax income associated with assets held for sale			35,675		52,378	83,418	109,019
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			$ 3,300		$ 15,300	$ 9,200	$ 30,100
Rate Reduction Bonds | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Amount of securitized rate reduction bonds issued								$ 635,700