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Receivables (Narrative) (Details) - Receivables Sold [Member] - IPL [Member] - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015
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Receivables [Line Items] |  |  |
Receivables sold to third party |  | $ 220.5 | $ 181.1
Cash proceeds of receivables sold | [1] | 62.0 | 5.0
Deferred proceeds |  | 154.2 | $ 172.0
Outstanding receivables past due |  | $ 47.5 |
[1] | Changes in cash proceeds are presented in “Sales of accounts receivable” in operating activities in Alliant Energy’s and IPL’s cash flows statements.

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