REGULATORY ACCOUNTING - Regulatory Tracker Mechanisms (Details) - Regulatory Tracker Deferrals Regulatory Assets [Member] $ in Millions	Dec. 31, 2016 USD ($)
The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Decoupling mechanisms permitted in recovery of an annual base amount	$ 1,059.0
Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Decoupling mechanisms permitted in recovery of an annual base amount	$ 132.4

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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