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Summary Of Significant Accounting Policies (Narrative) (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) customer MW | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Jan. 01, 2019 USD ($)
New Accounting Standards: |  |  |  |
Adjustment To Net Periodic Benefit Costs, Percentage | 40.00% |  |  |
Income statement impact of adopting new pension and other postretirement plans accounting standard |  | $ 17.8 | $ 17.1 |
Cash receipts on sold receivables | $ 605.3 | 461.8 | 466.8 |
IPL [Member] |  |  |  |
General: |  |  |  |
Generation and distribution of steam, number of customers served (in customers) | customer | 2 |  |  |
Property, Plant and Equipment: |  |  |  |
AFUDC accrual recorded, percentage of estimated CWIP | 100.00% |  |  |
New Accounting Standards: |  |  |  |
Income statement impact of adopting new pension and other postretirement plans accounting standard |  | 7.2 | 6.8 |
Cash receipts on sold receivables | $ 605.3 | 461.8 | 466.8 |
WPL [Member] |  |  |  |
Property, Plant and Equipment: |  |  |  |
AFUDC accrual recorded, percentage of estimated CWIP | 50.00% |  |  |
AFUDC rates, projects with approval | 100.00% |  |  |
New Accounting Standards: |  |  |  |
Income statement impact of adopting new pension and other postretirement plans accounting standard |  | 10.5 | 10.2 |
Alliant Energy Finance, LLC [Member] |  |  |  |
General: |  |  |  |
Ownership interest | 50.00% |  |  |
Electric capacity of wind farm (in megawatts) | MW | 225 |  |  |
Alliant Energy and IPL [Member] |  |  |  |
New Accounting Standards: |  |  |  |
Cash receipts on sold receivables |  | $ 461.8 | $ 466.8 |
Sheboygan Falls Energy Facility [Member] |  |  |  |
General: |  |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW | 347 |  |  |
Capital lease, lease term | 20 years |  |  |
Scenario, Forecast [Member] |  |  |  |
New Accounting Standards: |  |  |  |
Balance sheet impact of adopting new lease accounting standard |  |  |  | $ 18.0
Scenario, Forecast [Member] | IPL [Member] |  |  |  |
New Accounting Standards: |  |  |  |
Balance sheet impact of adopting new lease accounting standard |  |  |  | 11.0
Scenario, Forecast [Member] | WPL [Member] |  |  |  |
New Accounting Standards: |  |  |  |
Balance sheet impact of adopting new lease accounting standard |  |  |  | $ 7.0

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