RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	$ 6,569,900	$ 5,514,700
Regulatory Liabilities	3,850,781	3,658,042
Accrued Interest (included in Other Current Liabilities)	809,214	836,834
Rate Reduction Bonds - Current Portion	43,210	43,210
Rate Reduction Bonds - Long-Term Portion	496,912	540,122
Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	522,100	565,300
Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	161,000	125,400
Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	989,100	945,800
Regulatory Liabilities	383,183	413,381
Accrued Interest (included in Other Current Liabilities)	38,816	32,736
Rate Reduction Bonds - Current Portion	43,210	43,210
Rate Reduction Bonds - Long-Term Portion	496,912	540,122
Public Service Company of New Hampshire | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	522,100	565,300
Public Service Company of New Hampshire | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	20,900	14,700
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Rate Reduction Bonds - Current Portion	43,200	43,200
Rate Reduction Bonds - Long-Term Portion	496,900	540,100
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	522,100	565,300
Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Liabilities	9,100	5,600
Accrued Interest (included in Other Current Liabilities)	8,000	8,600
Special Deposits
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	73,600	52,500
Special Deposits | Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	36,800	32,500
Special Deposits | Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	36,800	32,500
Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	43,600	3,200
Other Long-Term Assets | Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash		3,200
Other Long-Term Assets | Variable Interest Entity, Primary Beneficiary | Public Service Company of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	$ 2,100	$ 3,200