Segments Of Business (Schedule Of Segments Of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Operating revenues	$ 897.4us-gaap_Revenues	$ 952.8us-gaap_Revenues
Operating income (loss)	152.9us-gaap_OperatingIncomeLoss	154.2us-gaap_OperatingIncomeLoss
Net income (loss) attributable to common shareowners	96.6	108.0
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	489.0	528.9
Operating income (loss)	65.5	57.5
Net income (loss) attributable to common shareowners	47.8	43.4
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	397.1	410.4
Operating income (loss)	80.8	87.7
Net income (loss) attributable to common shareowners	44.6	54.8
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	671.3	675.8
Operating income (loss)	104.7	94.3
Electric [Member] \| IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	362.6	374.2
Operating income (loss)	40.3	26.5
Electric [Member] \| WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	308.7	301.6
Operating income (loss)	64.4	67.8
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	198.4	240.7
Operating income (loss)	36.3	43.0
Gas [Member] \| IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	111.2	135.7
Operating income (loss)	20.6	23.3
Gas [Member] \| WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	87.2	105.0
Operating income (loss)	15.7	19.7
Other Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	16.4	22.8
Operating income (loss)	5.3	7.9
Other Utilities [Member] \| IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	15.2	19.0
Operating income (loss)	4.6	7.7
Other Utilities [Member] \| WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	1.2	3.8
Operating income (loss)	0.7	0.2
Utilities Business [Member]
Segment Reporting Information [Line Items]
Operating revenues	886.1	939.3
Operating income (loss)	146.3	145.2
Net income (loss) attributable to common shareowners	92.4	98.2
Non-Regulated [Member] \| Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	11.3	13.5
Operating income (loss)	6.6	9.0
Net income (loss) attributable to common shareowners	$ 4.2	$ 9.8

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

+ Details

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

- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

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

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ Details

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

- Details

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

- 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_PublicUtilityRegulatedOrUnregulatedStatusAxis=us-gaap_UnregulatedOperationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=lnt_ElectricMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=lnt_GasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=lnt_OtherUtilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=lnt_UtilityBusinessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: