SHORT-TERM CREDIT ARRANGEMENTS (Details) (USD $)	12 Months Ended													12 Months Ended						12 Months Ended
	Dec. 31, 2013 Gas Companies [Member]		Dec. 31, 2012 Gas Companies [Member]		Dec. 31, 2013 UIL [Member] Non-gas Companies [Member]		Dec. 31, 2012 UIL [Member] Non-gas Companies [Member]		Dec. 31, 2013 UI [Member] Non-gas Companies [Member]		Dec. 31, 2012 UI [Member] Non-gas Companies [Member]		Dec. 31, 2013 Standby Letter of Credit Due 2013 [Member]	Dec. 31, 2013 Standby Letter of Credit Due 2015 [Member] UIL [Member]	Dec. 31, 2013 Revolving Credit Facility [Member] UIL [Member]	Dec. 31, 2013 Revolving Credit Facility [Member] UI [Member]	Dec. 31, 2013 Revolving Credit Facility [Member] CNG [Member]	Dec. 31, 2013 Revolving Credit Facility [Member] SNG [Member]	Dec. 31, 2013 Revolving Credit Facility [Member] Berkshire [Member]	Dec. 31, 2013 Credit Agreement [Member] UIL [Member]	Oct. 31, 2012 Credit Agreement [Member] UIL [Member]
Disclosure of short-term credit arrangements [Abstract]
Available credit under the Credit Facility															$ 395,600,000	$ 250,000,000	$ 150,000,000	$ 150,000,000	$ 25,000,000
Amounts outstanding under the Credit Facility															0
Standby letter of credit outstanding													4,400,000
Expiration date														Jun. 16, 2015	Nov. 30, 2016					Oct. 31, 2013
Maximum borrowing capacity															400,000,000						100,000,000
Issuance of letter of credit, maximum															50,000,000
Short-term borrowings [Abstract]
Maximum aggregate principal amount of short-term borrowing outstanding at any month-end	0		6,000,000		202,000,000		168,000,000		60,000,000		243,000,000
Average aggregate short-term borrowings outstanding during the year	0	[1]	128,000	[1]	152,455,000	[1]	109,268,000	[1]	13,510,000	[1]	78,533,000	[1]
Weighted average interest rate (in hundredths)	0.00%	[1]	1.59%	[1]	1.43%	[1]	1.61%	[1]	1.40%	[1]	1.58%	[1]
Principal amounts outstanding at year-end	$ 0		$ 0		$ 0		$ 157,000,000		$ 0		$ 30,000,000

[1] Average short‑term borrowings represent the sum of daily borrowings outstanding, weighted for the number of days outstanding and divided by the number of days in the period. The weighted average interest rate is determined by dividing interest expense by the amount of average borrowings.