BUSINESS ORGANIZATION AND STATEMENT OF ACCOUNTING POLICIES, REGULATORY ASSETS (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 787,392		$ 779,962
Less current portion of regulatory assets	87,071		92,764
Regulatory Assets, Net	700,321		687,198
Unamortized Redemption Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	10,298		10,499
Remaining period, minimum	7 years
Remaining period, maximum	19 years
Pension and Other Post-Retirement Benefit Plans [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	403,358	[1]	402,700	[1]
Environmental Remediation Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	14,127		13,197
Remaining period	7 years
Hardship Programs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	19,661	[2]	24,744	[2]
Debt Premium [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	26,229		27,498
Remaining period, minimum	2 years
Remaining period, maximum	23 years
Income Taxes Due Principally to Book Tax Differences Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	166,039	[3]	164,466	[3]
Contracts For Differences [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	78,045	[4]	64,276	[4]
Term of contract minimum	5 years
Term of contract maximum	12 years
Deferred Transmission Income/Expense [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	20,981	[5]	17,387	[5]
Other [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	33,099	[6]	40,336	[6]
Unfunded Future Income Taxes [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 15,555	[7]	$ 14,859	[7]

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

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

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

[4] Asset life is equal to delivery term of related contracts (which vary from approximately 5 - 12 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.

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

[6] 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 certain amounts that are not currently earning a return; liability amount includes decoupling of $9.6 million.

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