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Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 counterparty
Derivative [Line Items]
Cash collateral posted by counterparty	$ 4.9
Number of counterparties exceeding derivative asset position (in counterparties)	1

X

- Definition

Number of Counterparties Exceeding Derivative Asset Position

+ References

No definition available.

+ Details

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Namespace Prefix:	lnt_
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X

- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959

+ Details

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Balance Type:	debit
Period Type:	instant

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- Details

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