RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Narrative (Details) - PSNH - Rate Reduction Bonds	May 31, 2018 USD ($)
Debt Instrument [Line Items]
Amount of securitized rate reduction bonds issued	$ 635,700,000
Weighted average interest rate	3.66%

Face (par) amount of debt instrument at time of issuance.

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Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant