COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) site	Dec. 31, 2015 USD ($) site	Dec. 31, 2014 USD ($)
Loss Contingencies [Line Items]
Environmental site quantity | site	61	64
Reserve	$ 65.8	$ 51.1	$ 43.3
Number of environmental sites designated as superfund sites | site	9
The Connecticut Light And Power Company [Member]
Loss Contingencies [Line Items]
Environmental site quantity | site	14	14
Reserve	$ 4.9	$ 4.6	3.8
Western Massachusetts Electric Company [Member]
Loss Contingencies [Line Items]
Environmental site quantity | site	4	4
Reserve	$ 0.6	$ 0.6	0.5
NSTAR Electric Company [Member]
Loss Contingencies [Line Items]
Environmental site quantity | site	13	15
Reserve	$ 3.2	$ 2.4	1.1
Number of environmental sites designated as superfund sites | site	4
Public Service Company Of New Hampshire [Member]
Loss Contingencies [Line Items]
Environmental site quantity | site	11	12
Reserve	$ 5.3	$ 4.5	$ 5.2
Number of environmental sites designated as superfund sites | site	3
MGP Site accrual [Member]
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites	$ 59.0	$ 45.5
Environmental Sites for Which a Range of Loss Exists [Member]
Loss Contingencies [Line Items]
Environmental site quantity | site	8
Reserve	$ 35.6
Best estimate of potential remediation costs	$ 16.0
Environmental Sites for Which a Range of Loss Exists [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies [Line Items]
Environmental site quantity | site	3
Reserve	$ 1.7
Best estimate of potential remediation costs	$ 1.0
Environmental Sites for Which a Range of Loss Exists [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies [Line Items]
Environmental site quantity | site	1
Reserve	$ 0.3
Environmental Site for Which a Range is Too Early to Determine [Member]
Loss Contingencies [Line Items]
Environmental site quantity | site	10
Reserve	$ 13.4
Environmental Site for Which a Range is Too Early to Determine [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies [Line Items]
Environmental site quantity | site	3
Reserve	$ 2.1
Evironmental Sites with Best Estimate [Member]
Loss Contingencies [Line Items]
Reserve	$ 16.8
Environmental site quantity remaining | site	43
Evironmental Sites with Best Estimate [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies [Line Items]
Reserve	$ 1.1
Environmental site quantity remaining | site	8
Evironmental Sites with Best Estimate [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies [Line Items]
Reserve	$ 0.3
Environmental site quantity remaining | site	3
Evironmental Sites with Best Estimate [Member] | NSTAR Electric Company [Member]
Loss Contingencies [Line Items]
Reserve	$ 3.2
Environmental site quantity remaining | site	13
Evironmental Sites with Best Estimate [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies [Line Items]
Reserve	$ 5.3
Environmental site quantity remaining | site	11
Superfund sites [Member]
Loss Contingencies [Line Items]
Best estimate of potential remediation costs	$ 0.7