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				9 Months Ended
	Sep. 30, 2014 Contract CapacityResource		Sep. 30, 2013		Sep. 30, 2014 Contract CapacityResource		Sep. 30, 2013		Sep. 30, 2014 Current Assets [Member]	Dec. 31, 2013 Current Assets [Member]	Sep. 30, 2014 Deferred Charges and Other Assets [Member]	Dec. 31, 2013 Deferred Charges and Other Assets [Member]	Sep. 30, 2014 Current Liabilities [Member]	Dec. 31, 2013 Current Liabilities [Member]	Sep. 30, 2014 Noncurrent Liabilities [Member]	Dec. 31, 2013 Noncurrent Liabilities [Member]	Sep. 30, 2014 Weather Insurance Contracts [Member] Southern Connecticut Gas Company (SCG) [Member]	Sep. 30, 2013 Weather Insurance Contracts [Member] Southern Connecticut Gas Company (SCG) [Member]	Sep. 30, 2014 Weather Insurance Contracts [Member] Berkshire [Member]	Oct. 31, 2013 Weather Insurance Contracts [Member] Berkshire [Member]
Derivative [Line Items]
Gross derivative asset	$ 28,800,000				$ 28,800,000				$ 6,861,000	$ 9,098,000	$ 21,945,000	$ 44,349,000
Regulatory asset	61,200,000				61,200,000
Gross derivative liability	83,400,000				83,400,000								23,404,000	26,976,000	59,948,000	169,327,000
Gross derivative asset, UIL's portion of entity	6,700,000				6,700,000
Gross derivative liability, UIL's portion of entity	55,700,000				55,700,000
Regulatory liabilities	6,600,000				6,600,000
Maximum receivable amount allowed under weather insurance contract																	3,000,000		1,000,000
Intrinsic value of contract																		2,000,000		200,000
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	393,000		(5,585,000)		(81,623,000)		(28,849,000)
Regulatory Liabilities - Derivative assets	5,077,000		0		(6,616,000)		0
Numerator [Abstract]
Net income attributable to UIL Holdings	12,498,000		5,144,000		77,293,000		74,859,000
Less: Net income allocated to unvested units	7,000		5,000		45,000		83,000
Net income attributable to common shareholders	$ 12,491,000		$ 5,139,000		$ 77,248,000		$ 74,776,000
Denominator [Abstract]
Basic average number of shares outstanding (in shares)	56,855		50,989		56,827		50,956
Effect of dilutive securities (in shares)	278	[1]	242	[1]	287	[1]	281	[1]
Diluted average number of shares outstanding (in shares)	57,133		51,231		57,114		51,237
Earnings per share [Abstract]
Basic (in dollars per share)	$ 0.22		$ 0.10		$ 1.36		$ 1.47
Diluted (in dollars per share)	$ 0.22		$ 0.10		$ 1.35		$ 1.46

[1] Includes unvested restricted stock and performance shares.