Regulatory Matters [Line Items]

No definition available.

Name:	lnt_RegulatoryMattersLineItems
Namespace Prefix:	lnt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash outflow to customers for refund payments of revenue that was previously reported as deferred or unearned revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 13.A.4(a).Q1 Response))
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

+ Details

Name:	us-gaap_DeferredRevenueRefundPayments1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(2))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateContinuingOperations
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The increase (decrease) during the reporting period in the value of other assets that are created when regulatory agencies permits public utilities to defer costs to the balance sheet.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet.

+ References

+ Details

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

- Definition

Amount of public utility's approved rate increase (decrease) by regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of public utility's approved rate increase (decrease) by regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesInterimRateIncreaseDecreaseAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesInterimRateIncreaseDecreasePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of public utility's amended requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmendedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of public utility's amended requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmendedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesRequestedRateIncreaseDecreasePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_RegulatoryAssetAmortizationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

+ Details

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

- Definition

The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

+ Details

Name:	us-gaap_RegulatoryLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_PublicUtilitiesRegulatoryProceedingAxis=lnt_IowaTaxReformMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=lnt_TwoThousandSeventeenTestYearRetailGasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=lnt_TwoThousandSixteenTestYearRetailElectricMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryLiabilityAxis=us-gaap_RevenueSubjectToRefundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=lnt_TwoThousandSeventeenandEighteenTestPeriodRetailElectricandGasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_IncomeTaxAuthorityAxis=us-gaap_StateAndLocalJurisdictionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=lnt_NetBookValueofMLKappMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=lnt_NetBookValueOfEdgewaterUnitFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2021	Dec. 31, 2019	Dec. 31, 2017
Regulatory Matters [Line Items]
State income tax rate	5.30%	13.20%	8.80%	14.90%
Decrease in regulatory assets			$ (27.4)	$ 108.9
Increase in regulatory liabilities			14.7	$ (64.8)
Regulatory assets	$ 1,694.7		1,694.7				$ 1,666.7
Refunded Federal Tax Reform benefits	20.0		47.0
Regulatory liabilities	$ 1,505.6		$ 1,505.6				1,497.2
IPL [Member]
Regulatory Matters [Line Items]
State income tax rate	(4.80%)	(13.20%)	(0.20%)	(9.80%)
Decrease in regulatory assets			$ (14.0)	$ 107.8
Increase in regulatory liabilities			7.5	$ (49.6)
Regulatory assets	$ 1,249.9		1,249.9				1,231.6
Refunded Federal Tax Reform benefits	9.0		16.0
Regulatory liabilities	$ 762.6		$ 762.6				755.4
WPL [Member]
Regulatory Matters [Line Items]
State income tax rate	11.80%	31.60%	14.20%	31.10%
Decrease in regulatory assets			$ (13.4)	$ 1.1
Regulatory assets	$ 444.8		444.8				435.1
Refunded Federal Tax Reform benefits	11.0		31.0
Regulatory liabilities	743.0		743.0				741.8
Iowa tax reform [Member] \| Alliant Energy and IPL [Member]
Regulatory Matters [Line Items]
Decrease in regulatory assets			33.7
Increase in regulatory liabilities			7.3
2017 Test Year Retail Gas [Member] \| IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase, amount	$ 11.0
Interim rate increase, percent	5.00%
2016 Test Year Retail Electric [Member] \| IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase, amount							$ 102.0
Interim rate increase, percent							7.00%
Approved rate increase, amount			$ 130.0
Approved rate increase, percent			9.00%
Scenario, Forecast [Member]
Regulatory Matters [Line Items]
Recovery period, authorized						10 years
Scenario, Forecast [Member] \| 2017 Test Year Retail Gas [Member] \| IPL [Member]
Regulatory Matters [Line Items]
Requested rate increase, amount						$ 20.0
Requested rate increase, percent						8.00%
Requested rate increase, amended amount						$ 14.0
Requested rate increase, amended percent						6.00%
Revenue Subject to Refund [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	$ 22.0		$ 22.0
Revenue Subject to Refund [Member] \| IPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	19.0		19.0
Revenue Subject to Refund [Member] \| WPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	3.0		3.0
Revenue Subject to Refund [Member] \| 2017 and 2018 Test Period Retail Electric and Gas [Member] \| Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Regulatory liabilities	12.0		$ 12.0
State [Member] \| IPL [Member]
Regulatory Matters [Line Items]
State income tax rate			12.00%
State [Member] \| Scenario, Forecast [Member] \| IPL [Member]
Regulatory Matters [Line Items]
State income tax rate					9.80%
Net book value of M.L. Kapp [Member] \| Alliant Energy and IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets	29.0		$ 29.0
Net book value of Edgewater Unit 4 [Member] \| Alliant Energy and WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets	$ 24.0		$ 24.0