BUSINESS ORGANIZATION AND STATEMENT OF ACCOUNTING POLICIES, DERIVATIVES, EARNINGS PER SHARE (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended													6 Months Ended
	Jun. 30, 2013 ContractforDifferences CapacityResources	Jun. 30, 2012	Jun. 30, 2013 ContractforDifferences CapacityResources	Jun. 30, 2012	Jun. 30, 2013 Current Assets [Member]	Dec. 31, 2012 Current Assets [Member]	Jun. 30, 2013 Deferred Charges and Other Assets [Member]	Dec. 31, 2012 Deferred Charges and Other Assets [Member]	Jun. 30, 2013 Current Liabilities [Member]	Dec. 31, 2012 Current Liabilities [Member]	Jun. 30, 2013 Noncurrent Liabilities [Member]	Dec. 31, 2012 Noncurrent Liabilities [Member]	Dec. 31, 2012 Berkshire [Member]	Jun. 30, 2013 Weather Insurance Contracts [Member] Southern Connecticut Gas Company (SCG) [Member]	Jun. 30, 2013 Weather Insurance Contracts [Member] Connecticut Natural Gas Corporation (CNG) [Member]	Jun. 30, 2013 Weather Insurance Contracts [Member] Berkshire [Member]
Derivative [Line Items]
Regulatory asset	$ 153,300,000		$ 153,300,000
Gross derivative liability, UIL's portion of entity	138,600,000		138,600,000
Payment received under weather insurance contract																1,000,000
Maximum receivable amount allowed under weather insurance contract														3,000,000	3,000,000
Maximum payable amount allowed under weather insurance contract														2,000,000	2,000,000
Weather insurance contracts													1,000,000
Derivative fair value, net [Abstract]
Gross derivative asset	58,700,000		58,700,000		9,380,000	12,671,000	49,320,000	67,167,000
Gross derivative liability	212,000,000		212,000,000						27,486,000	30,804,000	184,556,000	224,639,000
Derivatives [Abstract]
Number of new capacity resources selected	4		4
Number of Contracts for Differences (CfDs) approved by Public Utility Regulatory Authority (PURA)	4		4
Number of Contracts for Differences (CfDs) executed by UI	2		2
Number of Contracts for Differences (CfDs) executed by CL&P	2		2
Percentage of cost borne by UI customers for CFDs (in hundredths)			20.00%
Percentage of cost borne by CL&P customers (in hundredths)			80.00%
Unrealized gain and losses from mark-to-market adjustments [Abstract]
Regulatory Assets - Derivative liability	(15,099,000)	2,167,000	(23,263,000)	(5,781,000)
Regulatory Liabilities - Derivative assets	0	40,000	0	12,000
Numerator [Abstract]
Net income	17,924,000	11,999,000	69,715,000	59,049,000
Less: Net income allocated to unvested units	18,000	18,000	83,000	105,000
Net income attributable to common shareholders	$ 17,906,000	$ 11,981,000	$ 69,632,000	$ 58,944,000
Denominator [Abstract]
Basic average number of shares outstanding (in shares)	50,972	50,793	50,939	50,740
Effect of dilutive securities (in shares)	229	248	238	262
Diluted average number of shares outstanding (in shares)	51,201	51,041	51,177	51,002
Earnings per share [Abstract]
Basic (in dollars per share)	$ 0.35	$ 0.24	$ 1.37	$ 1.16
Diluted (in dollars per share)	$ 0.35	$ 0.23	$ 1.36	$ 1.16