REGULATORY PROCEEDINGS (Details) (Superstorm Sandy [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013 Customer
Superstorm Sandy [Member]
Catastrophic Event [Line Items]
Approximate number of customer outages	284,000
Estimate of costs to repair damages	$ 35.8
Estimate of costs to repair damages, amount capitalized	$ 13.9

The approximate capitalized cost to repair customer outages caused by extensive damage to the electric system in the company's service territory due to a natural disaster.

No definition available.

Name:	uil_EstimateOfCostsToRepairDamagesAmountCapitalized
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant