LONG-TERM DEBT - Long-Term Debt Issuances (Details) - USD ($)	Dec. 31, 2016	Jun. 30, 2016	May 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Eversource [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 (as a percentage)				2.50%
Eversource [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 (as a percentage)				3.35%
Eversource [Member] | Debentures [Member] | Series I Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	2.50%				2.50%
Eversource [Member] | Debentures [Member] | Series J Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.35%				3.35%
NSTAR Electric Company [Member] | Debentures [Member] | Unsecured Notes Due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument face amount			$ 250,000,000
Interest rate stated percentage (as a percentage)	2.70%		2.70%		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 (as a percentage)		2.75%