SHORT-TERM CREDIT ARRANGEMENTS (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012
SHORT-TERM CREDIT ARRANGEMENTS [Abstract]
Amounts outstanding under the Credit Facility, recorded as short-term debt	$ 95
Short-term Debt [Line Items]
Maximum borrowing capacity	105
Amount of pollution control revenue bonds required to repay and cancel as per use of funds under credit facility	103.5
UIL Standby Letter of Credit Due 2012 and 2013 [Member]
Short-term Debt [Line Items]
Standby letter of credit outstanding	4.3
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Available credit under the Credit Facility	$ 300.7