BUSINESS ORGANIZATION AND STATEMENT OF ACCOUNTING POLICIES, REGULATORY ASSETS (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 1,074,969		$ 1,135,450
Less current portion of regulatory assets	120,766		120,935
Regulatory Assets, Net	954,203		1,014,515
Remaining period	n/a
Nuclear Plant Investments - Above Market [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	242,275	[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	11,702		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	448,252	[2]	458,019	[2]
Environmental Remediation Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	15,006		14,772
Remaining period, minimum	4 years
Remaining period, maximum	5 years
Hardship Programs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	23,915	[3]	29,852	[3]
Debt Premium [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	37,452		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	21,545	[5]	17,319	[5]
Contracts For Differences [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	153,286	[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	14,967	[7]	8,864	[7]
Deferred Transmission Income/Expense [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	24,761	[8]	21,379	[8]
Storm Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	53,465	[9]	52,009	[9]
Other [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 28,343	[10]	$ 27,449	[10]

[1] Asset/Liability relates to the Competitive Transition Assessment (CTA). Total CTA costs recovery is currently projected to be completed in 2013, with stranded cost amortization expected to end in the fourth quarter of 2013. The remaining balances will be fully offset by amounts primarily included in income taxes, due principally to book-tax differences.

[2] Asset life is dependent upon timing of final pension plan distribution; balance 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 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).

[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. UI is seeking recovery of these costs in its rate proceeding discussed in Note (C) “Regulatory Proceedings – Electric Distribution and Transmission – Rates.”

[10] Amortization period and/or balance vary depending on the nature, cost of removal and/or remaining life of the underlying assets/liabilities.