REGULATORY PROCEEDINGS (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	6 Months Ended								3 Months Ended	6 Months Ended					6 Months Ended
	Dec. 31, 2011 MW	Jun. 30, 2012 MW	Jun. 29, 2012	Jun. 18, 2012	Apr. 30, 2012	Feb. 29, 2012	Jan. 31, 2012	Sep. 30, 2011	Jul. 31, 2010	Jun. 30, 2012 United Illuminating Company (UI) [Member]	Jun. 30, 2012 United Illuminating Company (UI) [Member]	Jun. 30, 2012 United Illuminating Company (UI) [Member] Maximum [Member]	Jun. 30, 2012 United Illuminating Company (UI) [Member] Minimum [Member]	Jun. 30, 2012 Connecticut Natural Gas Corporation (CNG) [Member]	Jun. 30, 2012 The Southern Connecticut Gas Company (SCG) [Member]	Jun. 30, 2012 The Berkshire Gas Company [Member]	Jun. 06, 2012 The Berkshire Gas Company [Member]
REGULATORY PROCEEDINGS [Abstract]
Percentage of Customers to establish standards for acceptable performance (in hundredths)		10.00%
Maximum number of hours to serve a customers (in hours)		48
Rates [Abstract]
Requested allowed distribution return on equity (in hundredths)		9.50%
Allowed distribution return on equity (in hundredths)		8.75%
Percentage of earnings returned to customers (in hundredths)		50.00%
Decoupling charge approved by PURA			$ 4.4
Approval for issuance of Debt [Abstract]
Principal amount of debt securities PURA approved for issuance		379
Principal amount of senior unsecured notes issued		303.5					103.5
Principal amount outstanding of pollution control revenue bonds remarketed						103.5
Additional principal amount of unsecured debt issued					100				100
Power Supply Arrangements [Abstract]
Percentage of standard service customers with wholesale power supply agreements in place for 2013 (in hundredths)										70.00%	70.00%
Collateral required to be posted if the entity's credit rating declined two ratings to fall below investment grade										9.3	9.3
New England East-West Solution [Abstract]
Number of inte related transmission projects										4	4
Number of inter-related transmission projects with portions sited in Connecticut										3	3
Minimum amount of entity's investment for ownership of specific transmission assets										60	60
Threshold percentage of CL&P's costs for the Connecticut portions of projects (in hundredths)										8.40%	8.40%
Expected amount of investment										60	60
Cumulative amount of deposits										13	13
Minimum period over which remaining investments are expected to be made											3 years
Maximum period over which remaining investments are expected to be made											5 years
Pre-tax income earned on each deposits in NEEWS project.										0.4	0.7
Weighted average return on equity (in hundredths)												12.40%	12.20%
Equity Investments in Peaking Generation [Abstract]
Approved return on equity (in hundredths)														9.41%	9.36%	10.50%
Duration of approved rate plan																10 years
Principal amount of debt securities DPU approved for issuance																	20
Maturity period of long term debt		30 years
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%	20.00%
Percentage of benefit from contract executed with CL&P (in hundredths)	20.00%	20.00%
Company is party to contracts related to project approved by PURA		2
Number of new renewable source generation megawatts having the capability of achieving commercial operation (in MW).		47
Contracts terminated		1
Maximum amount of commitment to purchase Renewable Energy Credits (RECs) from new facilities behind distribution customer meters		200
Period of commitment to purchase Renewable Energy Credits (RECs)		21 years
Solicitation period obligations will phase-in		6 years
Maximum annual commitment level obligation after year six		13.6
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
Renewable generation capacity for recovery (in MW)	10
Current approved base return on equity (in hundredths)								11.14%
Expected return on equity after reduction (in hundredths)								9.20%
Reduction in weighted average return on equity in basis points		25
Reduction in annual net income		$ 0.6
Number of solar facilities approved by PURA				1
Number of fuel cell facilities approved by PURA				2