BUSINESS ORGANIZATION AND STATEMENT OF ACCOUNTING POLICIES, REGULATORY ASSETS (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 1,008,187		$ 1,135,450
Less current portion of regulatory assets	98,323		120,935
Regulatory Assets, Net	909,864		1,014,515
Remaining period	n/a
Nuclear Plant Investments - Above Market [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	238,868	[1]	252,498	[1]
Connecticut Yankee [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	0		11,129
Unamortized Redemption Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	11,502		12,103
Remaining period, minimum	8 years
Remaining period, maximum	20 years
Pension and Other Post-Retirement Benefit Plans [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	444,038	[2]	458,019	[2]
Environmental Remediation Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	16,754		14,772
Remaining period, minimum	4 years
Remaining period, maximum	5 years
Hardship Programs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	23,833	[3]	29,852	[3]
Debt Premium [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	35,671		41,016
Remaining period, minimum	1 year
Remaining period, maximum	24 years
Deferred Purchased Gas [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	0	[4]	12,444	[4]
Unfunded Future Income Taxes [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	27,201	[5]	17,319	[5]
Contracts For Differences [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	147,726	[6]	176,597	[6]
Term of contract minimum	7 years
Term of contact maximum	14 years
Excess Generation Service Charge [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	8,228	[7]	8,864	[7]
Deferred Transmission Income/Expense [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	10,744	[8]	21,379	[8]
Storm Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	15,797	[9]	52,009	[9]
Other [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 27,825	[10]	$ 27,449	[10]

[1] Asset/Liability relates to the Competitive Transition Assessment (CTA). Total CTA costs recovery and stranded cost amortization were substantially completed in the third quarter of 2013. The remaining balances are fully offset by amounts primarily included in income taxes, due principally to book-tax differences. As a result of the outcome of UI’s 2013 distribution rate request, PURA approved UI’s proposed rate treatment to leave CTA rates unchanged until January 1, 2014 at which point the charge will end and the remaining balances will be extinguished.

[2] Asset life is dependent upon timing of final pension plan distribution; balance, which is fully offset by a corresponding liability, is recalculated each year in accordance with ASC 715 "Compensation-Retirement Benefits." See Note (G) “Pension and Other Benefits” for additional information.

[3] Hardship customer accounts deferred for future recovery to the extent they exceed the amount in rates.

[4] Deferred purchase gas costs balances at the end of the rate year are normally recorded/returned in the next year.

[5] The balance will be extinguished when the asset, which is fully offset by a corresponding liability, or liability has been realized or settled, respectively.

[6] Asset life is equal to delivery term of related contracts (which vary from approximately 7 - 14 years); balance fluctuates based upon quarterly market analysis performed on the related derivatives (Note K); amount, which does not earn a return, is fully offset by corresponding derivative asset/liability. See “-Contracts for Differences” discussion above for additional information.

[7] Working capital allowance for generation service charge; this amount fluctuates based upon cash inflows and outflows in a given period.

[8] Regulatory asset or liability which defers transmission income or expense and fluctuates based upon actual revenues and revenue requirements.

[9] Storm costs include accumulated costs for major storms occurring from January 2009 forward. See Note (C) “Regulatory Proceedings – Electric Distribution and Transmission – Rates” for a discussion of the recovery of these costs.

[10] Amortization period and/or balance vary depending on the nature, cost of removal and/or remaining life of the underlying assets/liabilities; asset amount includes decoupling ($12.4 million) and certain other amounts that are not currently earning a return. See Note (C) “Regulatory Proceedings – Electric Distribution and Transmission – Rates” for a discussion of the decoupling recovery period.