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Summary Of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Adjustment to net periodic benefit costs, percentage |  | 40.00% |
Income statement impact of adoption of Accounting Standards Update 2017-17 |  | $ 4.3 |
Cash receipts on sold receivables | $ 284.3 | $ 214.7 |
IPL [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Adjustment to net periodic benefit costs, percentage |  | 40.00% |
Income statement impact of adoption of Accounting Standards Update 2017-17 |  | $ 1.7 |
Cash receipts on sold receivables | $ 284.3 | $ 214.7 |
WPL [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Adjustment to net periodic benefit costs, percentage |  | 40.00% |
Income statement impact of adoption of Accounting Standards Update 2017-17 |  | $ 2.6 |
Alliant Energy and IPL [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Cash receipts on sold receivables |  | $ 214.7 | $ 681.9

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