CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Operating revenues	$ 1,682.3us-gaap_Revenues	$ 1,579.3us-gaap_Revenues	$ 1,499.2us-gaap_Revenues
Cost of fuel, natural gas, and purchased power	773.6us-gaap_CostOfGoodsAndServicesSold	722.5us-gaap_CostOfGoodsAndServicesSold	702.4us-gaap_CostOfGoodsAndServicesSold
Operating and maintenance expense	489.9us-gaap_OtherCostAndExpenseOperating	462.5us-gaap_OtherCostAndExpenseOperating	429.1us-gaap_OtherCostAndExpenseOperating
Depreciation and amortization expense	113.7	106.2	96.2
Taxes other than income taxes	46.7	48.1	47.3
Operating income	258.4us-gaap_OperatingIncomeLoss	240.0us-gaap_OperatingIncomeLoss	224.2us-gaap_OperatingIncomeLoss
Miscellaneous income	24.4us-gaap_OtherNonoperatingIncomeExpense	23.5us-gaap_OtherNonoperatingIncomeExpense	15.7us-gaap_OtherNonoperatingIncomeExpense
Interest expense	57.4us-gaap_InterestExpense	43.7us-gaap_InterestExpense	42.5us-gaap_InterestExpense
Other expense	(33.0)us-gaap_NonoperatingIncomeExpense	(20.2)us-gaap_NonoperatingIncomeExpense	(26.8)us-gaap_NonoperatingIncomeExpense
Income before taxes	225.4	219.8	197.4
Provision for income taxes	84.7us-gaap_IncomeTaxExpenseBenefit	81.9us-gaap_IncomeTaxExpenseBenefit	62.6us-gaap_IncomeTaxExpenseBenefit
Net income	140.7us-gaap_NetIncomeLoss	137.9us-gaap_NetIncomeLoss	134.8us-gaap_NetIncomeLoss
Preferred stock dividend requirements	(3.1)us-gaap_DividendsPreferredStockCash	(3.1)us-gaap_DividendsPreferredStockCash	(3.1)us-gaap_DividendsPreferredStockCash
Net income attributed to common shareholder	$ 137.6	$ 134.8	$ 131.7

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

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The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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Amount of paid and unpaid preferred stock dividends declared with the form of settlement in cash.

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses from ongoing operations, after income or loss from equity method investments, but before income taxes, extraordinary items, and noncontrolling interest.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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

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The total amount of other operating cost and expense items that are associated with the entity's normal revenue producing operation.

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The net amount of other income and expense amounts, the components of which are not separately disclosed on the income statement, resulting from ancillary business-related activities (that is, excluding major activities considered part of the normal operations of the business) also known as other nonoperating income (expense) recognized for the period. Such amounts may include: (a) dividends, (b) interest on securities, (c) net gains or losses on securities, (d) unusual costs, (e) gains or losses on foreign exchange transactions, and (f) miscellaneous other income and expense items.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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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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