COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) site gasSite	Dec. 31, 2019 USD ($) site	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Number of sites | site	63	57
Reserve	$ 102.4	$ 81.0	$ 64.7
Number Of Gas Sites Acquired | gasSite	9
The Connecticut Light and Power Company
Loss Contingencies [Line Items]
Number of sites | site	15	15
Reserve	$ 12.3	$ 11.4	5.4
NSTAR Electric Company
Loss Contingencies [Line Items]
Number of sites | site	12	15
Reserve	$ 4.7	$ 8.0	10.9
Public Service Company of New Hampshire
Loss Contingencies [Line Items]
Number of sites | site	9	9
Reserve	$ 7.1	$ 7.5	$ 5.4
MGP Site accrual
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites	$ 92.2	$ 67.9
Environmental Sites for Which a Range of Loss Exists
Loss Contingencies [Line Items]
Number of sites | site	6
Reserve	$ 41.7
Best estimate of potential remediation costs (up to)	$ 33.0
Environmental Sites for Which a Range of Loss Exists | The Connecticut Light and Power Company
Loss Contingencies [Line Items]
Number of sites | site	1
Reserve	$ 0.9
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	$ 17.5
Environmental Site for Which a Range is Too Early to Determine | The Connecticut Light and Power Company
Loss Contingencies [Line Items]
Number of sites | site	7
Reserve	$ 2.6
Environmental Site for Which a Range is Too Early to Determine | NSTAR Electric Company
Loss Contingencies [Line Items]
Number of sites | site	2
Reserve	$ 0.4
Evironmental Sites with Best Estimate
Loss Contingencies [Line Items]
Number of sites | site	42
Reserve	$ 43.2
Evironmental Sites with Best Estimate | The Connecticut Light and Power Company
Loss Contingencies [Line Items]
Reserve	$ 8.8
Environmental site quantity remaining | site	7
Evironmental Sites with Best Estimate | NSTAR Electric Company
Loss Contingencies [Line Items]
Reserve	$ 4.3
Environmental site quantity remaining | site	10
Evironmental Sites with Best Estimate | Public Service Company of New Hampshire
Loss Contingencies [Line Items]
Reserve	$ 7.1
Environmental site quantity remaining | site	9