Statements Of Operations (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Operating revenues	$ 163,746us-gaap_ElectricUtilityRevenue	$ 185,516us-gaap_ElectricUtilityRevenue	$ 895,755us-gaap_ElectricUtilityRevenue	$ 898,588us-gaap_ElectricUtilityRevenue
Energy expenses:
Fuel	37,729	51,586	237,148	233,955
Purchased and interchanged power	11,175	17,915	58,064	67,401
Cost of services, energy services	48,904us-gaap_CostOfServicesEnergyServices	69,501us-gaap_CostOfServicesEnergyServices	295,212us-gaap_CostOfServicesEnergyServices	301,356us-gaap_CostOfServicesEnergyServices
Operating revenues net of energy expenses	114,842ee_RevenuesNetEnergyExpenses	116,015ee_RevenuesNetEnergyExpenses	600,543ee_RevenuesNetEnergyExpenses	597,232ee_RevenuesNetEnergyExpenses
Other operating expenses:
Other operations	55,599	56,138	238,293	237,326
Maintenance	15,560	14,282	66,907	62,798
Depreciation and amortization	21,565	20,568	84,339	80,826
Taxes other than income taxes	14,158us-gaap_UtilitiesOperatingExpenseTaxes	15,362us-gaap_UtilitiesOperatingExpenseTaxes	61,546us-gaap_UtilitiesOperatingExpenseTaxes	60,327us-gaap_UtilitiesOperatingExpenseTaxes
Utilities operating expense	106,882us-gaap_UtilitiesOperatingExpense	106,350us-gaap_UtilitiesOperatingExpense	451,085us-gaap_UtilitiesOperatingExpense	441,277us-gaap_UtilitiesOperatingExpense
Operating income	7,960us-gaap_OperatingIncomeLoss	9,665us-gaap_OperatingIncomeLoss	149,458us-gaap_OperatingIncomeLoss	155,955us-gaap_OperatingIncomeLoss
Other income (deductions):
Allowance for equity funds used during construction	4,275	2,906	16,031	10,251
Investment and interest income, net	5,254ee_InterestAndOtherInvestmentIncomeLoss	4,241ee_InterestAndOtherInvestmentIncomeLoss	14,646ee_InterestAndOtherInvestmentIncomeLoss	10,043ee_InterestAndOtherInvestmentIncomeLoss
Miscellaneous non-operating income	180us-gaap_OtherNonoperatingIncome	1,517us-gaap_OtherNonoperatingIncome	2,738us-gaap_OtherNonoperatingIncome	2,425us-gaap_OtherNonoperatingIncome
Miscellaneous non-operating deductions	(491)us-gaap_OtherNonoperatingExpense	(419)us-gaap_OtherNonoperatingExpense	(4,271)us-gaap_OtherNonoperatingExpense	(3,583)us-gaap_OtherNonoperatingExpense
Nonoperating income (expense)	9,218us-gaap_NonoperatingIncomeExpense	8,245us-gaap_NonoperatingIncomeExpense	29,144us-gaap_NonoperatingIncomeExpense	19,136us-gaap_NonoperatingIncomeExpense
Interest charges (credits):
Interest on long-term debt and revolving credit facility	16,483	14,579	60,932	58,618
Other interest	163us-gaap_InterestExpenseOther	173us-gaap_InterestExpenseOther	1,240us-gaap_InterestExpenseOther	455us-gaap_InterestExpenseOther
Capitalized interest	(1,289)us-gaap_InterestCostsCapitalized	(1,246)us-gaap_InterestCostsCapitalized	(5,135)us-gaap_InterestCostsCapitalized	(5,243)us-gaap_InterestCostsCapitalized
Allowance for borrowed funds used during construction	(2,621)	(1,684)	(9,305)	(6,116)
Interest expense	12,736us-gaap_InterestExpense	11,822us-gaap_InterestExpense	47,732us-gaap_InterestExpense	47,714us-gaap_InterestExpense
Income before income taxes	4,442	6,088	130,870	127,377
Income tax expense	984us-gaap_IncomeTaxExpenseBenefit	1,473us-gaap_IncomeTaxExpenseBenefit	40,599us-gaap_IncomeTaxExpenseBenefit	41,813us-gaap_IncomeTaxExpenseBenefit
Net income	$ 3,458us-gaap_NetIncomeLoss	$ 4,615us-gaap_NetIncomeLoss	$ 90,271us-gaap_NetIncomeLoss	$ 85,564us-gaap_NetIncomeLoss
Basic earnings per share	$ 0.090us-gaap_EarningsPerShareBasic	$ 0.110us-gaap_EarningsPerShareBasic	$ 2.240us-gaap_EarningsPerShareBasic	$ 2.130us-gaap_EarningsPerShareBasic
Diluted earnings per share	$ 0.090us-gaap_EarningsPerShareDiluted	$ 0.110us-gaap_EarningsPerShareDiluted	$ 2.240us-gaap_EarningsPerShareDiluted	$ 2.120us-gaap_EarningsPerShareDiluted
Dividends declared per share of common stock	$ 0.280	$ 0.265	$ 1.120	$ 1.060
Weighted average number of shares outstanding	40,243,199	40,149,083	40,214,198	40,132,106
Weighted average number of shares and dilutive potential shares outstanding	40,266,674	40,149,083	40,240,793	40,144,159

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This item represents investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities, and income or expense derived from the amortization of investment related discounts or premiums, respectively, net of related investment expenses. This item includes realized or unrealized gains or losses on the sale or holding of investments in debt and equity securities required to be included in earnings for the period or for other than temporary losses related to investments in debt and equity securities which are included in realized losses in the period recognized, and does not include investment income from real or personal property, such as rental income. Also, includes other interest income from operating activities.