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Benefit Plans (Defined Contribution Retirement Plans) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
401(k) costs	$ 6.0	$ 5.7	$ 12.5	$ 11.9
IPL [Member]
401(k) costs	3.1	2.9	6.5	6.0
WPL [Member]
401(k) costs	$ 2.6	$ 2.6	$ 5.5	$ 5.4

X

- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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

X

- Details

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

X

- Details

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