COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) site	Dec. 31, 2018 USD ($) site	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Number of sites | site	57	60
Reserve	$ 81.0	$ 64.7	$ 54.9
CL&P
Loss Contingencies [Line Items]
Number of sites | site	15	15
Reserve	$ 11.4	$ 5.4	4.7
NSTAR Electric
Loss Contingencies [Line Items]
Number of sites | site	15	16
Reserve	$ 8.0	$ 10.9	2.7
PSNH
Loss Contingencies [Line Items]
Number of sites | site	9	9
Reserve	$ 7.5	$ 5.4	$ 5.7
MGP Site accrual
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites	$ 67.9	$ 50.1
Environmental Sites for Which a Range of Loss Exists
Loss Contingencies [Line Items]
Number of sites | site	6
Reserve	$ 35.9
Best estimate of potential remediation costs (up to)	$ 35.0
Environmental Sites for Which a Range of Loss Exists | CL&P
Loss Contingencies [Line Items]
Number of sites | site	1
Reserve	$ 0.4
Best estimate of potential remediation costs (up to)	$ 0.5
Environmental Site for Which a Range is Too Early to Determine
Loss Contingencies [Line Items]
Number of sites | site	15
Reserve	$ 7.4
Environmental Site for Which a Range is Too Early to Determine | CL&P
Loss Contingencies [Line Items]
Number of sites | site	7
Reserve	$ 2.3
Environmental Site for Which a Range is Too Early to Determine | NSTAR Electric
Loss Contingencies [Line Items]
Number of sites | site	4
Reserve	$ 1.9
Evironmental Sites with Best Estimate
Loss Contingencies [Line Items]
Number of sites | site	36
Reserve	$ 37.7
Evironmental Sites with Best Estimate | CL&P
Loss Contingencies [Line Items]
Reserve	$ 8.7
Environmental site quantity remaining | site	7
Evironmental Sites with Best Estimate | NSTAR Electric
Loss Contingencies [Line Items]
Reserve	$ 6.1
Environmental site quantity remaining | site	11
Evironmental Sites with Best Estimate | PSNH
Loss Contingencies [Line Items]
Reserve	$ 7.5
Environmental site quantity remaining | site	9