COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details) - USD ($) $ in Millions			9 Months Ended		12 Months Ended	15 Months Ended
	Apr. 29, 2016	Mar. 22, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2014	Jul. 31, 2017	Oct. 31, 2015	Mar. 27, 2014
FERC ROE Complaints [Member]
Loss Contingencies [Line Items]
Base ROE percentage challenged by complainants			11.14%
Period of complaint			15 months
Loss Contingency, loss in period				$ 20.0
FERC ROE First Complaint [Member]
Loss Contingencies [Line Items]
Base ROE percentage of first complaint period					10.57%
FERC ROE First Complaint [Domain]
Loss Contingencies [Line Items]
Maximum percentage cap of ROE for incentive projects					11.74%
FERC ROE Second and Third Complaints [Member]
Loss Contingencies [Line Items]
Base ROE percentage challenged by complainants			11.14%
FERC ROE Second Complaint [Member]
Loss Contingencies [Line Items]
Period of complaint			15 months					15 months
FERC ALJ recommended base ROE percentage		9.59%
Basis point change			10.00%
Loss contingency, estimate of possible earnings impact			$ 3.0
FERC ROE Third Complaint [Member]
Loss Contingencies [Line Items]
Period of complaint			15 months				15 months
FERC ALJ recommended base ROE percentage		10.90%
Loss contingency, estimate of possible earnings impact			$ 3.0
FERC ROE Fourth Complaint [Member]
Loss Contingencies [Line Items]
Base ROE percentage challenged by complainants	10.57%
Maximum percentage cap of ROE for incentive projects	11.74%
The Connecticut Light And Power Company [Member] | FERC ROE Complaints [Member]
Loss Contingencies [Line Items]
Loss Contingency, loss in period				12.5
NSTAR Electric Company [Member] | FERC ROE Complaints [Member]
Loss Contingencies [Line Items]
Loss Contingency, loss in period				2.4
Public Service Company Of New Hampshire [Member] | FERC ROE Complaints [Member]
Loss Contingencies [Line Items]
Loss Contingency, loss in period				1.0
Western Massachusetts Electric Company [Member] | FERC ROE Complaints [Member]
Loss Contingencies [Line Items]
Loss Contingency, loss in period				$ 4.1
Minimum [Member] | FERC ROE Second Complaint [Member]
Loss Contingencies [Line Items]
FERC ALJ recommended zone of reasonableness FERC ROE percentage		7.12%
Potential loss from base ROE percentage			10.57%					10.57%
Minimum [Member] | FERC ROE Third Complaint [Member]
Loss Contingencies [Line Items]
FERC ALJ recommended zone of reasonableness FERC ROE percentage		7.04%
Maximum [Member] | FERC ROE Second Complaint [Member]
Loss Contingencies [Line Items]
FERC ALJ recommended zone of reasonableness FERC ROE percentage		10.42%
Potential loss from base ROE percentage								9.59%
Estimate of after-tax increase to reserve			$ 34.0
Maximum [Member] | FERC ROE Third Complaint [Member]
Loss Contingencies [Line Items]
FERC ALJ recommended zone of reasonableness FERC ROE percentage		12.19%
Estimate of after-tax decrease to reserve			$ 8.0
Subsequent Event [Member] | FERC ROE Fourth Complaint [Member]
Loss Contingencies [Line Items]
Period of complaint						15 months