SHORT-TERM AND LONG-TERM DEBT - Schedule of Issuances and Repayments (Details) - USD ($)	1 Months Ended			3 Months Ended
	Jan. 31, 2015	May 31, 2013	May 31, 2008	Mar. 31, 2018	Mar. 31, 2017	Jan. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Repayments				$ (150,218,000)	$ (150,000,000)
The Connecticut Light And Power Company
Debt Instrument [Line Items]
Repayments				0	$ (150,000,000)
The Connecticut Light And Power Company | Senior Notes | 4.00% 2018 Series A First Mortgage Bonds
Debt Instrument [Line Items]
Issuances				$ 500,000,000
Interest rate, stated percentage				4.00%
The Connecticut Light And Power Company | Senior Notes | 2008 Series A First Mortgage Bonds Due 2018
Debt Instrument [Line Items]
Repayments			$ (300,000,000)
Interest rate, stated percentage			5.65%
Public Service Company Of New Hampshire | Senior Notes | 2008 Series O First Mortgage Bonds Due 2018
Debt Instrument [Line Items]
Repayments			$ (110,000,000)
Interest rate, stated percentage			6.00%
Eversource Parent | Senior Notes | Eversource Parent 2.50% Series I Senior Notes
Debt Instrument [Line Items]
Issuances						$ 200,000,000.0
Long-term debt							$ 450,000,000
Interest rate, stated percentage						2.50%
Eversource Parent | Senior Notes | 3.30% Series M Senior Notes
Debt Instrument [Line Items]
Issuances						$ 450,000,000.0
Interest rate, stated percentage						3.30%
Eversource Parent | Senior Notes | Eversource Parent 1.60% Series G Senior Notes
Debt Instrument [Line Items]
Repayments	$ (150,000,000)
Interest rate, stated percentage	1.60%
Eversource Parent | Senior Notes | Series E Senior Notes Due 2018
Debt Instrument [Line Items]
Repayments		$ (300,000,000)
Interest rate, stated percentage		1.45%