REGULATORY PROCEEDINGS, PART TWO (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended											6 Months Ended
	Feb. 15, 2013	Jun. 30, 2013	Jun. 30, 2013 United Illuminating Company (UI) [Member] Megawatt	Jun. 30, 2012 United Illuminating Company (UI) [Member]	Jun. 30, 2013 United Illuminating Company (UI) [Member] Megawatt	Jun. 30, 2012 United Illuminating Company (UI) [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] Megawatt	Jul. 30, 2013 United Illuminating Company (UI) [Member]	Apr. 04, 2013 United Illuminating Company (UI) [Member]	Feb. 28, 2013 United Illuminating Company (UI) [Member]	Dec. 31, 2012 United Illuminating Company (UI) [Member]	Sep. 30, 2012 United Illuminating Company (UI) [Member]	Apr. 30, 2012 United Illuminating Company (UI) [Member]	Jan. 31, 2012 United Illuminating Company (UI) [Member]	Jul. 31, 2010 United Illuminating Company (UI) [Member]	Jun. 30, 2013 United Illuminating Company (UI) [Member] Maximum Transmission [Member]	Jun. 30, 2013 United Illuminating Company (UI) [Member] Minimum Transmission [Member]	Jun. 30, 2013 Connecticut Natural Gas Corporation (CNG) [Member]	Jun. 30, 2013 The Southern Connecticut Gas Company (SCG) [Member]	Jun. 30, 2013 The Berkshire Gas Company [Member]	Jun. 30, 2013 GenConn Devon [Member]	Jun. 30, 2013 GenConn Middletown [Member]
Rates [Abstract]
Length of time for proposed rate amendment	2 years
Revenues for past incurred storm costs included in proposed additional revenues	$ 53
Proposed additional revenues for past incurred storm costs, recovery period	6 years
Proposed additional revenues to establish storm reserve per year	2
Schedule of Regulatory Proceedings [Line Items]
Allowed distribution return on equity (in hundredths)			8.75%		8.75%
Allowed distribution return on equity per 2013 draft decision (in hundredths)								9.15%
Percentage of earnings retained by company (in hundredths)			50.00%		50.00%
Reduction in regulatory asset for major storm costs per PURA draft decision in the 2013 rate case								16
Amount of capital costs disallowed								21
Potential pre-tax write-off								37
Decoupling charge to be approved by PURA			4.4		4.4				6.4
Approval for issuance of Debt [Abstract]
Principal amount of debt securities PURA approved for issuance			379		379
Principal Amount Of Pollution Control Revenue Bonds			103.5		103.5
Principal amount of senior unsecured notes issued			303.5		303.5
Debt included in principal amount of senior unsecured notes issued													100.0	103.5	100.0
Power Supply Arrangements [Abstract]
Percentage of standard service customers with wholesale power supply agreements in place for 2013 (in hundredths)					100.00%
Percentage of standard service customers with wholesale power supply agreements in place for 2014 (in hundredths)					30.00%
Collateral required to be posted if the entity's credit rating declined two ratings to fall below investment grade			8.2		8.2
New Renewable Source Generation [Abstract]
Approximate amount of required commitment to purchase Renewable Energy Credits (RECs) from new facilities behind distribution customer meters			200		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		13.6
Total amount of annual payments related to REC purchase contracts entered into by UI			2.1		2.1
Delivery terms of REC purchase contracts (in years)					15 years
Amount of annual payments on contracts commencing in 2014			2		2
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
Mega watts of solar projects selected by DEEP (in MW)							5
Percentage of cost from contract executed with CL&P (in hundredths)					20.00%
Percentage of benefit from contract executed with CL&P (in hundredths)					20.00%
Renewable generation capacity for recovery (in MW)			10		10
Requested ROE, by UI (in hundredths)			9.50%
Percentage of UIs distribution load for which UI may be directed to enter to contracts for energy and/or RECs (in hundredths)		4.00%
New England East-West Solution [Abstract]
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			34		34
Amount of transmission assets										18.4		6.2
Pre-tax income earned on each deposit in NEEWS project.			0.3	0.4	0.9	0.7
Equity Investment in Peaking Generation [Abstract]
Ratio of equity investment in peaking generation for joint venture			50-50		50-50
Revenue requirements for equity investment in peaking generation																					33.1	40.2
Transmission Return of Equity (ROE) [Abstract]
Weighted average return on equity (in hundredths)																12.40%	12.20%
Current approved base return on equity (in hundredths)			11.14%		11.14%
Return on equity sought by complainants (in hundredths)			9.20%		9.20%
Revised return on equity sought by complainants (in hundredths)			8.90%		8.90%
Revised return on equity supported by municipal utilities (in hundredths)			8.50%		8.50%
Revised return on equity supported by FERC (in hundredths)			8.93%		8.93%
Change in weighted average return on equity in basis points					25
Change in annual net income			0.6		0.6
Refund period					15 months
Requested ROE by New England transmission owners and UI											8.70%
Rates [Abstract]
Duration of approved rate plan																				10 years
Approved return on equity (in hundredths)																		9.41%	9.36%	10.50%
Proposed ROE (in hundredths)																		10.25%
Additional annual base delivery rate revenues																		$ 19.7
Incremental revenues above existing rate schedule (in hundredths)																		6.30%