COMMITMENTS AND CONTINGENCIES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Loss Contingencies [Line Items]
Sales price of Electric System Work Center (ESWC)	$ 10.2
Original purchase price of ESWC	16
Middletown/Norwalk Transmission Project [Member]
Loss Contingencies [Line Items]
Amount sought for change order requests	33.3
Revised amount sought for change order requests	8.4
Percentage of general contractor mark-up on approved subcontractor change order claims (in hundredths)	10.00%
Amount of general contractor mark-up on approved subcontractor change order claims	2.3
GenConn Devon Facility [Member]
Loss Contingencies [Line Items]
Settlement amount paid	10.5
GenConn Middletown Facility [Member]
Loss Contingencies [Line Items]
Settlement amount paid	3.0
Positive Outcome of Litigation DOE Spent Fuel Litigation [Member]
Gain Contingencies [Line Items]
Damages awarded for spent fuel-related costs	39.7
Entity's share of amount awarded to atomic power company	3.8
Second set of damage claims relating to failure to remove spent atomic fuel	135
Entity's share of second set of damage claims	12.8
Mill River New Haven Connecticut Site [Member]
Site Contingency [Line Items]
Amount placed in escrow for estimated remediation	1.9
Amount of escrow fund unexpended	0.1
Connecticut Yankee Atomic Power Company [Member]
Site Contingency [Line Items]
Percentage stock ownership share in atomic power company (in hundredths)	9.50%
Carrying value of stock in atomic power company	0.2
Regulatory asset related to atomic power company	14.2
New Haven Harbor Station Site [Member]
Site Contingency [Line Items]
Site contingency, liability accrued	3.2
Pine Street Bridgeport Connecticut Site [Member]
Site Contingency [Line Items]
Site contingency, liability accrued	2.0
Minimum range of future remediation costs	2
Maximum range of future remediation costs	3
Mill Street Greenfield Massachusetts Site [Member]
Site Contingency [Line Items]
Site contingency, liability accrued	5.0
Low range of threshold for future expenses	5.0
East Street Site Pittsfield Massachusetts [Member]
Site Contingency [Line Items]
Site contingency, liability accrued	2.7
Amount demanded as share of response costs	$ 1.1