RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Summary of the Impact of Funding on the Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Costs (included in Regulatory Assets)	$ 4,830,800	$ 5,145,900
Regulatory Liabilities	3,644,477	3,609,475
Accrued Interest (included in Other Current Liabilities)	806,839	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 Costs (included in Regulatory Assets)	576,100	608,400
Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Costs (included in Regulatory Assets)	131,800	208,000
Prepayments and Other Current Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	37,700	72,100
Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	71,900	3,200
PSNH
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Costs (included in Regulatory Assets)	870,400	929,500
Regulatory Liabilities	418,106	428,069
Accrued Interest (included in Other Current Liabilities)	61,260	64,046
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 Costs (included in Regulatory Assets)	576,100	608,400
PSNH | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Costs (included in Regulatory Assets)	13,000	12,100
PSNH | Prepayments and Other Current Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	18,500	47,500
PSNH | Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	3,200	3,200
PSNH | VIE
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
PSNH | VIE | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Costs (included in Regulatory Assets)	576,100	608,400
PSNH | VIE | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Liabilities	7,700	5,800
Accrued Interest (included in Other Current Liabilities)	3,400	14,400
PSNH | VIE | Prepayments and Other Current Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	18,500	47,500
PSNH | VIE | Other Long-Term Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	$ 3,200	$ 3,200