REGULATORY PROCEEDINGS (Details) (USD $)	12 Months Ended									12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 United Illuminating Company (UI) [Member]	Feb. 06, 2012 United Illuminating Company (UI) [Member]	Jan. 30, 2012 United Illuminating Company (UI) [Member]	Aug. 31, 2011 United Illuminating Company (UI) [Member]	Sep. 01, 2010 United Illuminating Company (UI) [Member]	Dec. 31, 2009 United Illuminating Company (UI) [Member]	Dec. 31, 2011 Connecticut Natural Gas Corporation (CNG) [Member]	Dec. 31, 2011 The Southern Connecticut Gas Company (SCG) [Member]	Dec. 31, 2011 The Berkshire Gas Company [Member]	Dec. 31, 2011 The Gas Companies [Member]
Tropical Storm Irene [Abstract]
Percentage of customers without power due to the autumn nor'easter (in hundredths)				16.00%
Percentage of customers without power due to Tropical Storm Irene (in hundredths)				59.00%
Cost to repair damage and restore electrical service				$ 25,000,000
Capitalized portion of cost to repair damage and restore electrical service				5,000,000
Rates [Abstract]
Allowed distribution return on equity (in hundredths)									8.75%
Previously approved allowed distribution return on equity (in hundredths)									9.75%
Capital structure, equity ratio (in hundredths)									50.00%
Capital structure, debt ratio (in hundredths)									50.00%
Previously approved capital structure equity ratio (in hundredths)									48.00%
Previously approved capital structure debt ratio (in hundredths)									52.00%
Percentage of earnings returned to customers (in hundredths)									50.00%
Percentage of earnings retained by the entity (in hundredths)									50.00%
Duration of pilot program (in years)				2
Decoupling charge approved by PURA							1,400,000	1,600,000
Pension and earnings sharing over-recovery credit approved by the PURA								3,600,000
Duration of decoupling adjustment recovery period	twelve month period beginning in October 2011			twelve month period commencing in October 2010
Power Supply Arrangements [Abstract]
Maximum demand of customers who did not choose a retail electric supplier						less than 500 kilowatts
Percentage of standard service customers with wholesale power supply agreements in place for 2013 (in hundredths)				40.00%
Collateral required to be posted if the entity's credit rating declined two ratings to fall below investment grade				12,200,000
Transmission Return on Equity (ROE) [Abstract]
Current base return on equity on transmission investments (in hundredths)				11.14%
New England East-West Solution [Abstract]
Number of inter-related transmission projects				4
Number of inter-related transmission projects with portions sited in Connecticut				3
Percentage of transmission construction work in progress costs of which recovery of return in authorized (in hundredths)				100.00%
Minimum amount of entity's investment for ownership of specific transmission assets				60,000,000
Threshold percentage of CL&P's costs for the Connecticut portions of projects (in hundredths)				8.40%
Expected amount of investment				60,000,000
Cumulative amount of deposits				9,600,000
Additional amount of deposits made in NEEWS					1,300,000
Minimum period over which remaining investments are expected to be made (in years)				3
Maximum period over which remaining investments are expected to be made (in years)				5
Pre-tax income earned on each deposits in NEEWS project.				1,000,000
Equity Investments in Peaking Generation [Abstract]
Ownership percentage in joint venture (in hundredths)	50.00%			50.00%
Number of new peaking generation plants				2
Approved 2012 revenue requirements for GenConn Devon facility				34,000,000
Approved 2012 revenue requirements for the GenConn Middletown facility				42,300,000
Approved return on equity (in hundredths)										9.41%	9.36%	10.50%
Duration of approved rate plan (in years)												10Y
Recoverable carrying charges										2,200,000	2,200,000
Approval for issuance of Debt [Abstract]
Face amount of secured medium-term notes approved for issuance											50,000,000
Maximum fixed coupon rate (in hundredths)											7.00%
Maturity of debt approved for issuance, minimum (in years)											1Y
Maturity of debt approved for issuance, maximum (in years)											40Y
Principal amount of maturing existing debt				103,500,000							30,000,000
Issued long term debt Southern Connecticut Gas											50,000,000
Principal amount of debt securities PURA approved for issuance				379,000,000
Principal amount of senior unsecured notes issued	103,500,000			203,500,000
Additional principal amount of unsecured debt issued				100,000,000
New Renewable Source Generation [Abstract]
Statutory requirements to purchase output of new renewable generation (in MW)				150
Sharing agreement executed with CL&P to share cost (in hundredths)				20.00%
Percentage of benefit from contract executed with CL&P (in hundredths)				20.00%
Number of projects approved by PURA				7
Company is party to contracts related to project approved by PURA				2
Duration of contract (in years)				15Y
Percentage of delivered products purchased and accounted for as operating lease (in hundredths)				100.00%
Percentage of delivered products purchased and accounted for on accrual basis (in hundredths)				84.50%
Capacity of delivered products (in MW)				4.8
Additional fuel cell projects approved by PURA				5
Number of new renewable source generation megawatts having the capability of achieving commercial operation (in MW).				47
Maximum amount of commitment to purchase Renewable Energy Credits (RECs) from new facilities behind distribution customer meters				200,000,000
Period of commitment to purchase Renewable Energy Credits (RECs) (in years)				21Y
Solicitation period obligations will phase-in (in years)				6Y
Maximum annual commitment level obligation after year six				13,600,000
Number of mega watts of grid connected renewable energy allowed to be developed per Connecticut Public Act 11-80 (in MW)				30
Number of mega watts of grid connected renewable energy allowed to be developed by each company (in MW)				10
Amount of grid connected renewable energy solicited by DEEP from market				10
Number of solar projects selected by DEEP				2
Mega watts of solar projects selected by DEEP (in MW)				5
Number of transportation contracts on twelve different pipelines				89
Number of pipelines connected to Gas Companies' distribution system				3
Number of Contracts held with six different storage contractors				24
ISO-NE and RTO-NE [Abstract]
Basis point ROE adder on Pool Transmission Facilities (PTF) for participation in the RTO-NE				50
Basis point ROE incentive for projects included in the ISO-NE Regional System Plan				100
Weighted-average ROE for its transmission business (in hundredths)				12.40%
Minimum purchase obligation [Abstract]
2012													116,086,000
2013													111,169,000
2014													104,823,000
2015													84,538,000
2016													71,639,000
2017- after													136,694,000
Total													624,949,000
Pension and Postretirement Expenses [Abstract]
Period for recovery of pension and postretirement expenses approved (in years)			4Y
Amount of regulatory asset allowed to be recorded	1,100,000		10,200,000
Amount included in rates for a portion increased pension and postretirement expense		$ 11,400,000