COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) site	Dec. 31, 2016 USD ($) site	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Environmental site quantity | site	59	61
Reserve | $	$ 54.9	$ 65.8	$ 51.1
Number of environmental sites designated as superfund sites | site	9
The Connecticut Light And Power Company
Loss Contingencies [Line Items]
Environmental site quantity | site	14	14
Reserve | $	$ 4.7	$ 4.9	4.6
NSTAR Electric Company
Loss Contingencies [Line Items]
Environmental site quantity | site	15	17
Reserve | $	$ 2.7	$ 3.8	3.0
Number of environmental sites designated as superfund sites | site	4
Public Service Company Of New Hampshire
Loss Contingencies [Line Items]
Environmental site quantity | site	10	11
Reserve | $	$ 5.7	$ 5.3	$ 4.5
Number of environmental sites designated as superfund sites | site	3
MGP Site accrual
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites | $	$ 49.0	$ 59.0
Environmental Sites for Which a Range of Loss Exists
Loss Contingencies [Line Items]
Environmental site quantity | site	8
Reserve | $	$ 25.4
Best estimate of potential remediation costs (up to) | $	$ 20.0
Environmental Sites for Which a Range of Loss Exists | The Connecticut Light And Power Company
Loss Contingencies [Line Items]
Environmental site quantity | site	3
Reserve | $	$ 1.8
Environmental Sites for Which a Range of Loss Exists | NSTAR Electric Company
Loss Contingencies [Line Items]
Environmental site quantity | site	1
Reserve | $	$ 0.3
Environmental Site for Which a Range is Too Early to Determine
Loss Contingencies [Line Items]
Environmental site quantity | site	10
Reserve | $	$ 12.3
Environmental Site for Which a Range is Too Early to Determine | The Connecticut Light And Power Company
Loss Contingencies [Line Items]
Environmental site quantity | site	3
Reserve | $	$ 1.8
Evironmental Sites with Best Estimate
Loss Contingencies [Line Items]
Reserve | $	$ 17.2
Environmental site quantity remaining | site	41
Evironmental Sites with Best Estimate | The Connecticut Light And Power Company
Loss Contingencies [Line Items]
Reserve | $	$ 1.1
Environmental site quantity remaining | site	8
Evironmental Sites with Best Estimate | NSTAR Electric Company
Loss Contingencies [Line Items]
Reserve | $	$ 2.4
Environmental site quantity remaining | site	14
Evironmental Sites with Best Estimate | Public Service Company Of New Hampshire
Loss Contingencies [Line Items]
Reserve | $	$ 5.7
Environmental site quantity remaining | site	10
Superfund sites [Member]
Loss Contingencies [Line Items]
Best estimate of potential remediation costs (up to) | $	$ 0.9