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Debt (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended |  |
--- | --- | --- | ---
Oct. 31, 2017 | Jul. 31, 2017 | Dec. 31, 2017 | Sep. 30, 2017
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity |  |  |  | $ 1,000.0
Parent Company [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity |  |  |  | 300.0
IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity |  |  |  | 300.0
WPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity |  |  |  | 400.0
Alliant Energy and IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Commercial paper, long-term |  |  |  | $ 40.0
Commercial paper, long-term, interest rate |  |  |  | 1.40%
Term Loan Credit Agreement [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Proceeds from issuance of debt |  | $ 95.0 |  |
Variable-rate term loan credit agreement, period |  | 364 days |  |
Interest rate |  |  |  | 1.80%
Subsequent Event [Member] | 3.05% debenture, due 2027 [Member] | Debentures [Member] | WPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Proceeds from issuance of debt | $ 300.0 |  |  |
Interest rate | 3.05% |  |  |
Scenario, Forecast [Member] | Parent Company [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  | $ 500.0 |
Scenario, Forecast [Member] | IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  | 400.0 |
Scenario, Forecast [Member] | WPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  | $ 500.0 |

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + 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 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + 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(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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