RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	$ 5,514,700	$ 5,145,900
Regulatory Liabilities	3,658,042	3,609,475
Accrued Interest (included in Other Current Liabilities)	836,834	823,006
Rate Reduction Bonds - Current Portion	43,210	52,332
Rate Reduction Bonds - Long-Term Portion	540,122	583,331
Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	565,300	608,400
Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	125,400	208,000
PSNH
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	945,800	929,500
Regulatory Liabilities	413,381	428,069
Accrued Interest (included in Other Current Liabilities)	32,736	44,093
Rate Reduction Bonds - Current Portion	43,210	52,332
Rate Reduction Bonds - Long-Term Portion	540,122	583,331
PSNH | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	565,300	608,400
PSNH | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	14,700	12,100
Variable Interest Entity, Primary Beneficiary | PSNH
Condensed Balance Sheet Statements, Captions [Line Items]
Rate Reduction Bonds - Current Portion	43,200	52,300
Rate Reduction Bonds - Long-Term Portion	540,100	583,300
Variable Interest Entity, Primary Beneficiary | PSNH | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	565,300	608,400
Variable Interest Entity, Primary Beneficiary | PSNH | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Liabilities	5,600	5,800
Accrued Interest (included in Other Current Liabilities)	8,600	14,400
Prepayments and Other Current Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	52,500	72,100
Prepayments and Other Current Assets | PSNH
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash		47,500
Prepayments and Other Current Assets | Variable Interest Entity, Primary Beneficiary | PSNH
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	32,500	47,500
Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	3,200	3,200
Other Long-Term Assets | PSNH
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash		3,200
Other Long-Term Assets | Variable Interest Entity, Primary Beneficiary | PSNH
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	$ 3,200	$ 3,200