SHORT-TERM AND LONG-TERM DEBT - Schedule of Commercial Paper Program (Details) - Commercial Paper - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
NSTAR Electric Company
Debt Instrument [Line Items]
Borrowings Outstanding	$ 443.8	$ 234.0
Available Borrowing Capacity	$ 206.2	$ 416.0
Weighted-Average Interest Rate	1.99%	1.55%
Eversource Parent
Debt Instrument [Line Items]
Borrowings Outstanding	$ 747.0	$ 979.3
Available Borrowing Capacity	$ 703.0	$ 470.7
Weighted-Average Interest Rate	2.28%	1.86%

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Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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