STATEMENT OF ACCOUNTING POLICIES, DERIVATIVES, EARNINGS PER SHARE (Details) (USD $)	3 Months Ended																12 Months Ended																	12 Months Ended
	Dec. 31, 2013 CapacityResources		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013 CapacityResources	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Current Assets [Member]	Dec. 31, 2012 Current Assets [Member]	Dec. 31, 2013 Deferred Charges and Other Assets [Member]	Dec. 31, 2012 Deferred Charges and Other Assets [Member]	Dec. 31, 2013 Current Liabilities [Member]	Dec. 31, 2012 Current Liabilities [Member]	Dec. 31, 2013 Noncurrent Liabilities [Member]	Dec. 31, 2012 Noncurrent Liabilities [Member]	Dec. 31, 2013 Weather Insurance Contracts [Member] Southern Connecticut Gas Company (SCG) [Member]	Sep. 30, 2013 Weather Insurance Contracts [Member] Southern Connecticut Gas Company (SCG) [Member]	Oct. 31, 2012 Weather Insurance Contracts [Member] Southern Connecticut Gas Company (SCG) [Member]	Dec. 31, 2013 Weather Insurance Contracts [Member] Connecticut Natural Gas Corporation (CNG) [Member]	Oct. 31, 2013 Weather Insurance Contracts [Member] Connecticut Natural Gas Corporation (CNG) [Member]	Oct. 31, 2012 Weather Insurance Contracts [Member] Connecticut Natural Gas Corporation (CNG) [Member]	Dec. 31, 2013 Weather Insurance Contracts [Member] Berkshire [Member]	Oct. 31, 2013 Weather Insurance Contracts [Member] Berkshire [Member]	Dec. 31, 2012 Weather Insurance Contracts [Member] Berkshire [Member]
Derivative [Line Items]
Gross derivative asset	$ 53,400,000																$ 53,400,000			$ 9,098,000	$ 12,671,000	$ 44,349,000	$ 67,167,000
Regulatory asset	142,700,000																142,700,000
Gross derivative liability	196,200,000																196,200,000							26,976,000	30,804,000	169,327,000	224,639,000
Gross derivative liability, UIL's portion of entity	129,400,000																129,400,000
Payment received under weather insurance contract																																		1,000,000
Maximum receivable amount allowed under weather insurance contract																													3,000,000	3,000		1,500,000	3,000,000		1,000,000
Maximum payable amount allowed under weather insurance contract																													2,000,000	2,000		1,000,000	2,000,000		200,000
Intrinsic value of contract																												0			100,000			0		1,000,000
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 or benefit borne by or allocated to UI customers for CFDs (in hundredths)																	20.00%
Percentage of cost or benefit borne by or allocated to CL&P customers (in hundredths)																	80.00%
Unrealized gain and losses from mark-to-market adjustments [Abstract]
Regulatory Assets - Derivative liabilities																	(33,819,000)	(7,500,000)
Regulatory Liabilities - Derivative assets																	0	12,000
Numerator [Abstract]
Net income attributable to UIL Holdings	40,406,000		5,144,000		17,924,000		51,791,000		28,839,000		15,749,000		11,999,000		47,050,000		115,265,000	103,637,000	99,656,000
Less: Net income allocated to unvested units																	117,000	184,000	210,000
Net income attributable to common shareholders																	$ 115,148,000	$ 103,453,000	$ 99,446,000
Denominator [Abstract]
Basic average number of shares outstanding (in shares)																	52,415,000	50,831,000	50,609,000
Effect of dilutive securities (in shares)																	296,000	277,000	317,000
Diluted average number of shares outstanding (in shares)																	52,711,000	51,108,000	50,926,000
Earnings per share [Abstract]
Basic (in dollars per share)	$ 0.71	[1]	$ 0.10	[1]	$ 0.35	[1]	$ 1.02	[1]	$ 0.57	[1]	$ 0.31	[1]	$ 0.24	[1]	$ 0.93	[1]	$ 2.20	$ 2.04	$ 1.96
Diluted (in dollars per share)	$ 0.71	[1]	$ 0.10	[1]	$ 0.35	[1]	$ 1.01	[1]	$ 0.56	[1]	$ 0.31	[1]	$ 0.23	[1]	$ 0.92	[1]	$ 2.18	$ 2.02	$ 1.95
Number of shares of common stock outstanding with purchase options not included in diluted earnings per share (in shares)																			89,336

[1] Based on weighted average number of shares outstanding each quarter. Diluted earnings per share reflects the effect of dilutive stock options, performance shares and restricted stock. Quarterly per share amounts are not intended to be additive.