RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Summary of the Impact of Funding on the Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Assets	$ 5,096,700	$ 5,145,900
Regulatory Liabilities	3,627,389	3,609,475
Accrued Interest (included in Other Current Liabilities)	840,587	823,006
Rate Reduction Bonds - Current Portion	43,210	52,332
Rate Reduction Bonds - Long-Term Portion	561,727	583,331
Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Assets	597,700	608,400
Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Assets	202,200	208,000
Prepayments and Other Current Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	45,900	72,100
Long-term Derivative Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	3,200	3,200
PSNH
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Assets	932,000	929,500
Regulatory Liabilities	429,932	428,069
Accrued Interest (included in Other Current Liabilities)	55,088	64,046
Rate Reduction Bonds - Current Portion	43,210	52,332
Rate Reduction Bonds - Long-Term Portion	561,727	583,331
PSNH | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Assets	597,700	608,400
PSNH | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Assets	11,900	12,100
PSNH | Prepayments and Other Current Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	21,100	47,500
PSNH | Long-term Derivative 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	561,700	583,300
PSNH | VIE | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Assets	597,700	608,400
PSNH | VIE | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Liabilities	10,000	5,800
Accrued Interest (included in Other Current Liabilities)	3,500	14,400
PSNH | VIE | Prepayments and Other Current Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	21,100	47,500
PSNH | VIE | Long-term Derivative Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash	$ 3,200	$ 3,200