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Leases (Schedule Of Lease Expenses Included In Consolidated Statements Of Income) (Details) (WPL [Member], Sheboygan Falls Energy Facility [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
WPL [Member] | Sheboygan Falls Energy Facility [Member]
Property, Plant and Equipment [Line Items]
Interest expense	$ 10.4us-gaap_CapitalLeasesIncomeStatementInterestExpense / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember	$ 10.9us-gaap_CapitalLeasesIncomeStatementInterestExpense / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember	$ 11.3us-gaap_CapitalLeasesIncomeStatementInterestExpense / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember
Depreciation and amortization	6.2us-gaap_CapitalLeasesIncomeStatementAmortizationExpense / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember	6.2us-gaap_CapitalLeasesIncomeStatementAmortizationExpense / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember	6.2us-gaap_CapitalLeasesIncomeStatementAmortizationExpense / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember
Total lease expenses included in income statements	$ 16.6lnt_CapitalLeasesIncomeStatementLeaseExpenses / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember	$ 17.1lnt_CapitalLeasesIncomeStatementLeaseExpenses / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember	$ 17.5lnt_CapitalLeasesIncomeStatementLeaseExpenses / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_SheboyganFallsEnergyFacilityMember

X

- Definition

Capital Leases, Income Statement, Lease Expenses

+ References

No definition available.

+ Details

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Namespace Prefix:	lnt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The charge for the use of long-lived depreciable capital leased assets for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45031-112735



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 840

-Section 55

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501752&loc=d3e56872-110439

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The financing charge for leasing arrangements meeting the criteria for capitalization.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45031-112735

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration