CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
CURRENT ASSETS
Cash and cash equivalents	$ 1.5	$ 5.5
Accounts receivable, less reserve of $1.2 and $1.6, respectively	169.2us-gaap_AccountsReceivableNetCurrent	188.8us-gaap_AccountsReceivableNetCurrent
Accounts receivable - unconsolidated affiliates	6.6	5.6
Accrued unbilled revenues	46.5us-gaap_UnbilledReceivablesCurrent	55.5us-gaap_UnbilledReceivablesCurrent
Income taxes receivable	15.1us-gaap_IncomeTaxesReceivable	16.0us-gaap_IncomeTaxesReceivable
Fuel inventories	82.6us-gaap_InventoryNet	58.5us-gaap_InventoryNet
Materials and supplies, at average cost	78.7us-gaap_OtherInventorySupplies	78.9us-gaap_OtherInventorySupplies
Deferred income taxes	189.2us-gaap_DeferredTaxAssetsNetCurrent	191.4us-gaap_DeferredTaxAssetsNetCurrent
Fuel clause under recoveries	27.0us-gaap_DeferredFuelCost	68.3us-gaap_DeferredFuelCost
Other	36.9us-gaap_OtherAssetsCurrent	37.3us-gaap_OtherAssetsCurrent
Total current assets	653.3us-gaap_AssetsCurrent	705.8us-gaap_AssetsCurrent
OTHER PROPERTY AND INVESTMENTS
Investment in unconsolidated affiliates	1,315.7us-gaap_EquityMethodInvestments	1,318.2us-gaap_EquityMethodInvestments
Other	71.4us-gaap_OtherLongTermInvestments	70.1us-gaap_OtherLongTermInvestments
Total other property and investments	1,387.1us-gaap_OtherInvestments	1,388.3us-gaap_OtherInvestments
PROPERTY, PLANT AND EQUIPMENT
In service	10,020.9	9,983.0
Construction work in progress	147.5us-gaap_ConstructionInProgressGross	115.9us-gaap_ConstructionInProgressGross
Total property, plant and equipment	10,168.4us-gaap_PropertyPlantAndEquipmentGross	10,098.9us-gaap_PropertyPlantAndEquipmentGross
Less accumulated depreciation	3,156.2	3,119.0
Net property, plant and equipment	7,012.2us-gaap_PropertyPlantAndEquipmentNet	6,979.9us-gaap_PropertyPlantAndEquipmentNet
DEFERRED CHARGES AND OTHER ASSETS
Regulatory assets	406.7us-gaap_RegulatoryAssetsNoncurrent	411.5us-gaap_RegulatoryAssetsNoncurrent
Other	42.9	42.3
Total deferred charges and other assets	449.6us-gaap_OtherAssetsNoncurrent	453.8us-gaap_OtherAssetsNoncurrent
TOTAL ASSETS	9,502.2us-gaap_Assets	9,527.8us-gaap_Assets
CURRENT LIABILITIES
Short-term debt	109.9us-gaap_ShortTermBorrowings	98.0us-gaap_ShortTermBorrowings
Accounts payable	151.9us-gaap_AccountsPayableCurrent	179.1us-gaap_AccountsPayableCurrent
Dividends payable	49.9us-gaap_DividendsPayableCurrent	49.9us-gaap_DividendsPayableCurrent
Customer deposits	75.2us-gaap_CustomerDepositsCurrent	73.7us-gaap_CustomerDepositsCurrent
Accrued taxes	24.0us-gaap_TaxesPayableCurrent	39.7us-gaap_TaxesPayableCurrent
Accrued interest	34.2us-gaap_InterestPayableCurrent	43.0us-gaap_InterestPayableCurrent
Accrued compensation	33.6	38.2
Long-term debt due within one year	110.0us-gaap_LongTermDebtCurrent	0us-gaap_LongTermDebtCurrent
Other	52.9us-gaap_OtherLiabilitiesCurrent	51.7us-gaap_OtherLiabilitiesCurrent
Total current liabilities	641.6us-gaap_LiabilitiesCurrent	573.3us-gaap_LiabilitiesCurrent
LONG-TERM DEBT	2,645.4us-gaap_LongTermDebtNoncurrent	2,755.3us-gaap_LongTermDebtNoncurrent
DEFERRED CREDITS AND OTHER LIABILITIES
Accrued benefit obligations	313.2	315.5
Deferred income taxes	2,279.3	2,268.3
Regulatory liabilities	271.6us-gaap_RegulatoryLiabilityNoncurrent	263.0us-gaap_RegulatoryLiabilityNoncurrent
Other	109.3us-gaap_OtherLiabilitiesNoncurrent	108.0us-gaap_OtherLiabilitiesNoncurrent
Total deferred credits and other liabilities	2,973.4	2,954.8
Total liabilities	6,260.4us-gaap_Liabilities	6,283.4us-gaap_Liabilities
COMMITMENTS AND CONTINGENCIES (NOTE 12)
STOCKHOLDERS' EQUITY
Common stockholders' equity	1,091.5	1,087.6
Retained earnings	2,191.5	2,198.2
Accumulated other comprehensive loss, net of tax	(41.2)	(41.4)
Total stockholders' equity	3,241.8us-gaap_StockholdersEquity	3,244.4us-gaap_StockholdersEquity
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 9,502.2	$ 9,527.8

Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

Name:	us-gaap_AccountsPayableCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant