SHORT-TERM AND LONG-TERM DEBT - Commercial Paper Programs and Credit Agreements (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Short-term debt reclassified to long-term debt	$ 898,800,000
Eversource Parent
Line of Credit Facility [Line Items]
Amount outstanding during period	0	$ 0
Eversource Parent | Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity	1,450,000,000
Short-term borrowings outstanding	917,000,000	1,000,000,000
Remaining borrowing capacity	$ 533,000,000	$ 428,000,000
Weighted average interest rate (as a percentage)	1.34%	0.88%
Eversource Parent | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,450,000,000
Debt instrument term	5 years
The Connecticut Light And Power Company
Line of Credit Facility [Line Items]
Notes payable to Eversource Parent	$ 0	$ 80,100,000
Public Service Company Of New Hampshire
Line of Credit Facility [Line Items]
Notes payable to Eversource Parent	202,300,000	160,900,000
Western Massachusetts Electric Company
Line of Credit Facility [Line Items]
Notes payable to Eversource Parent	96,900,000	51,000,000
NSTAR Electric Company
Line of Credit Facility [Line Items]
Amount outstanding during period	0	0
NSTAR Electric Company | Commercial Paper
Line of Credit Facility [Line Items]
Maximum borrowing capacity	450,000,000
Short-term borrowings outstanding	0	126,500,000
Remaining borrowing capacity	450,000,000	$ 323,500,000
Weighted average interest rate (as a percentage)		0.71%
NSTAR Electric Company | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 450,000,000
Debt instrument term	5 years