SHORT-TERM AND LONG-TERM DEBT - Long-Term Debt Issuances (Details) - USD ($)	Jun. 30, 2016	May 31, 2016	Mar. 31, 2016
Eversource Parent [Member] | Senior Notes [Member] | Series I Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount			$ 250,000,000
Interest rate stated percentage			2.50%
Eversource Parent [Member] | Senior Notes [Member] | Series J Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount			$ 250,000,000
Interest rate stated percentage			3.35%
NSTAR Electric Company [Member] | Unsecured Debt [Member] | Unsecured Notes Due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 250,000,000
Interest rate stated percentage		2.70%
Western Massachusetts Electric Company [Member] | Senior Notes [Member] | Series H Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 50,000,000
Interest rate stated percentage	2.75%