Jointly-Owned Electric Utility Plant (Details) $ in Millions	Dec. 31, 2015 USD ($)
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 1,526.7
Accumulated Provision for Depreciation	501.2
Construction Work in Progress	41.8
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	852.1
Accumulated Provision for Depreciation	267.5
Construction Work in Progress	6.9
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	674.6
Accumulated Provision for Depreciation	233.7
Construction Work in Progress	$ 34.9
Ottumwa Unit 1 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	$ 481.4
Accumulated Provision for Depreciation	125.0
Construction Work in Progress	$ 6.5
George Neal Unit 4 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	$ 184.3
Accumulated Provision for Depreciation	76.3
Construction Work in Progress	$ 0.2
George Neal Unit 3 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	$ 150.0
Accumulated Provision for Depreciation	45.1
Construction Work in Progress	$ 0.1
Louisa Unit 1 [Member] \| IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	$ 36.4
Accumulated Provision for Depreciation	21.1
Construction Work in Progress	$ 0.1
Columbia Units 1-2 [Member] \| WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	46.20%
Electric Plant	$ 571.9
Accumulated Provision for Depreciation	175.5
Construction Work in Progress	$ 34.8
Edgewater Unit 4 [Member] \| WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	68.20%
Electric Plant	$ 102.7
Accumulated Provision for Depreciation	58.2
Construction Work in Progress	$ 0.1

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

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ 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_OttumwaUnit1Member
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_GeorgeNealUnit3Member
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_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: