DERIVATIVE INSTRUMENTS - Gross Fair Value of Contracts Narrative (Details) $ in Millions	Dec. 31, 2016 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative assets subject to credit risk with investment grade counterparty	$ 76.2
The Connecticut Light And Power Company [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative assets subject to credit risk with investment grade counterparty	$ 70.0

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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