Debt (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended
Jun. 30, 2018	Apr. 30, 2018
Alliant Energy and IPL [Member]
Debt [Line Items]
Commercial paper, long-term	$ 125.0
Commercial paper, long-term, weighted average interest rate	2.30%
Term Loan Credit Agreement [Member] \| Short-term loan credit agreement through 2018 [Member] \| Alliant Energy Finance LLC [Member]
Debt [Line Items]
Retirement of debt	$ 95.0
Term Loan Credit Agreement [Member] \| Term loan credit agreement through April 2020 [Member] \| Alliant Energy Finance LLC [Member]
Debt [Line Items]
Proceeds from issuance of long-term debt		$ 300.0
Variable-rate term loan, interest rate	2.60%
Term Loan Credit Agreement [Member] \| Term loan credit agreement through 2018 [Member] \| Alliant Energy Finance LLC [Member]
Debt [Line Items]
Retirement of debt	$ 500.0
Senior Notes [Member] \| 3.75% senior notes due 2023 [Member] \| Alliant Energy Finance LLC [Member]
Debt [Line Items]
Proceeds from issuance of long-term debt	$ 400.0
Variable-rate term loan, interest rate	3.75%
Senior Notes [Member] \| 4.25% senior notes due 2028 [Member] \| Alliant Energy Finance LLC [Member]
Debt [Line Items]
Proceeds from issuance of long-term debt	$ 300.0
Variable-rate term loan, interest rate	4.25%

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20,22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

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