Jointly-Owned Electric Utility Plant (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 1,488.1
Accumulated Provision for Depreciation	486.8
Construction Work in Progress	22.4
Cost Of Removal Obligations Included In Regulatory Liabilities	45.3
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	842.6
Accumulated Provision for Depreciation	264.6
Construction Work in Progress	3.2
Cost Of Removal Obligations Included In Regulatory Liabilities	32.6
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	645.5
Accumulated Provision for Depreciation	222.2
Construction Work in Progress	19.2
Cost Of Removal Obligations Included In Regulatory Liabilities	12.7
Ottumwa Unit 1 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	488.7
Accumulated Provision for Depreciation	131.6
Construction Work in Progress	1.4
Cost Of Removal Obligations Included In Regulatory Liabilities	11.0
George Neal Unit 4 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	183.3
Accumulated Provision for Depreciation	71.3
Construction Work in Progress	0.3
Cost Of Removal Obligations Included In Regulatory Liabilities	12.3
George Neal Unit 3 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	135.2
Accumulated Provision for Depreciation	41.3
Construction Work in Progress	1.4
Cost Of Removal Obligations Included In Regulatory Liabilities	6.0
Louisa Unit 1 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	35.4
Accumulated Provision for Depreciation	20.4
Construction Work in Progress	0.1
Cost Of Removal Obligations Included In Regulatory Liabilities	3.3
Columbia Units 1-2 [Member] \| WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	46.20%
Electric Plant	549.0
Accumulated Provision for Depreciation	166.8
Construction Work in Progress	18.9
Cost Of Removal Obligations Included In Regulatory Liabilities	10.2
Edgewater Unit 4 [Member] \| WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	68.20%
Electric Plant	96.5
Accumulated Provision for Depreciation	55.4
Construction Work in Progress	0.3
Cost Of Removal Obligations Included In Regulatory Liabilities	$ 2.5

Jointly Owned Utility Cost Of Removal Obligations Included In Regulatory Liabilities

No definition available.

Name:	lnt_JointlyOwnedUtilityCostOfRemovalObligationsIncludedInRegulatoryLiabilities
Namespace Prefix:	lnt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 10.C)

-URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033

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

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section C

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

+ References

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

+ References

+ Details

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

- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=lnt_ColumbiaUnits1And2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=lnt_EdgewaterUnit4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=lnt_GeorgeNealUnit3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=lnt_GeorgeNealUnit4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=lnt_LouisaUnit1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=lnt_OttumwaUnit1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: