RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory assets	$ 5,153,300	$ 5,239,300
Accrued Interest (included in Other Current Liabilities)	616,662	594,176
Rate Reduction Bonds - Current Portion	52,332	0
Rate Reduction Bonds - Long-Term Portion	583,331	0
Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory assets	618,600	0
Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory assets	169,200	137,400
Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory assets	953,500	940,800
Accrued Interest (included in Other Current Liabilities)	34,790	33,437
Rate Reduction Bonds - Current Portion	52,332	0
Rate Reduction Bonds - Long-Term Portion	583,331	0
Public Service Company of New Hampshire | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory assets	618,600	0
Public Service Company of New Hampshire | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory assets	12,400	$ 15,800
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash - Current Portion (included in Prepayments and Other Current Assets)	26,900
Restricted Cash - Long-Term Portion (included in Other Long-Term Assets)	3,200
Accrued Interest (included in Other Current Liabilities)	8,800
Rate Reduction Bonds - Current Portion	52,300
Rate Reduction Bonds - Long-Term Portion	583,300
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory assets	$ 1,000