SHORT-TERM AND LONG-TERM DEBT - Long-Term Debt Issuances (Details) - Senior Notes [Member] - USD ($)	May 31, 2017	Mar. 31, 2017
Eversource Parent [Member] | Series K Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 300,000,000
Interest rate stated percentage		2.75%
The Connecticut Light And Power Company [Member] | 2017 Series A First and Refunding Mortgage Bonds Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 300,000,000
Interest rate stated percentage		3.20%
NSTAR Electric Company [Member] | Unsecured Notes Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 350,000,000
Interest rate stated percentage	3.20%