Condensed Consolidated Statements Of Income (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Condensed Consolidated Statements Of Income [Abstract]
UTILITY OPERATING REVENUES	$ 332,201us-gaap_ElectricUtilityRevenue	$ 355,303us-gaap_ElectricUtilityRevenue
UTILITY OPERATING EXPENSES:
Fuel	85,432	104,098
Other operating expenses	54,976us-gaap_UtilitiesOperatingExpenseOther	62,119us-gaap_UtilitiesOperatingExpenseOther
Power purchased	42,576	37,748
Maintenance	31,282	31,973
Depreciation and amortization	46,445	46,055
Taxes other than income taxes	12,933	12,208
Income taxes - net	18,201us-gaap_CurrentIncomeTaxExpenseBenefit	16,559us-gaap_CurrentIncomeTaxExpenseBenefit
Total utility operating expenses	291,845us-gaap_UtilitiesOperatingExpense	310,760us-gaap_UtilitiesOperatingExpense
UTILITY OPERATING INCOME	40,356us-gaap_OperatingIncomeLoss	44,543us-gaap_OperatingIncomeLoss
OTHER INCOME AND (DEDUCTIONS):
Allowance for equity funds used during construction	3,218	1,613
Miscellaneous income and (deductions) - net	(626)us-gaap_OtherNonoperatingIncomeExpense	(975)us-gaap_OtherNonoperatingIncomeExpense
Income tax benefit applicable to nonoperating income	6,024	5,754
Total other income and (deductions) - net	8,616us-gaap_NonoperatingIncomeExpense	6,392us-gaap_NonoperatingIncomeExpense
INTEREST AND OTHER CHARGES:
Interest on long-term debt	27,646us-gaap_InterestExpenseLongTermDebt	26,440us-gaap_InterestExpenseLongTermDebt
Other interest	449us-gaap_InterestExpenseOther	453us-gaap_InterestExpenseOther
Allowance for borrowed funds used during construction	(2,666)	(983)
Amortization of redemption premiums and expense on debt	1,334	1,319
Total interest and other charges - net	26,763us-gaap_InterestCostsIncurred	27,229us-gaap_InterestCostsIncurred
NET INCOME	22,209us-gaap_NetIncomeLoss	23,706us-gaap_NetIncomeLoss
LESS: PREFERRED DIVIDENDS OF SUBSIDIARY	803	803
NET INCOME APPLICABLE TO COMMON STOCK	$ 21,406	$ 22,903

Income tax benefit applicable to non operating income.

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Amount of noncash expense included in interest expense to allocate debt discount and premium, and the costs to issue debt and obtain financing over the related debt instruments. Alternate captions include noncash interest expense.

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The aggregate revenue, whether regulated or unregulated, derived from the generation, transmission and distribution of electricity.

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Aggregate amount of interest paid or due on all long-term debt.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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The aggregate amount of income or expense from ancillary business-related activities (that is to say, excluding major activities considered part of the normal operations of the business).

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The net result for the period of deducting operating expenses from operating revenues.

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The amount of preferred stock dividends that is an adjustment to net income apportioned to common stockholders.

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The component of the allowance for funds used during construction during the period based on an assumed rate of return on equity funds used in financing the construction of regulated assets.

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Amount capitalized of allowance for funds used during construction.

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All taxes not related to income of the entity or excise or sales taxes levied on the revenue of the entity that are not reported elsewhere. These taxes could include production, real estate, personal property, and pump tax.

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Amount of operating expense of regulated operation.

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Amount of operating expense for depreciation and amortization of regulated operation.

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Amount of operating expense for fuel used by regulated operation.

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Amount of operating expense for routine plant maintenance and repairs of regulated operation.

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Amount of operating expense for purchased power of regulated operation.

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