Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Money market fund investments	$ 18.2	$ 128.3
Derivative assets	25.1	20.9
Foreign exchange contracts	0.4	4.8
Deferred proceeds	87.4	152.9
Available-for-sale securities	1.8	2.1
Derivative liabilities	52.4	67.3
Foreign exchange contracts		0.1
IPL [Member] \| Level 1 [Member]
Money market fund investments		5.6
Available-for-sale securities	0.8	0.6
WPL [Member] \| Level 1 [Member]
Money market fund investments	18.2
Level 1 [Member]
Money market fund investments	18.2	128.3
Available-for-sale securities	0.8	0.6
IPL [Member] \| Level 2 [Member]
Derivative assets	2.9	3.6
Available-for-sale securities	0.9	1.4
IPL [Member] \| Level 2 [Member] \| Commodity Contracts [Member]
Derivative assets	2.9	3.6
Derivative liabilities	19.3	23.8
WPL [Member] \| Level 2 [Member]
Derivative liabilities	28.0	40.1
WPL [Member] \| Level 2 [Member] \| Commodity Contracts [Member]
Derivative assets	4.8	6.4
Derivative liabilities	28.0	40.1
Level 2 [Member]
Derivative assets	7.7	10.0
Available-for-sale securities	0.9	1.4
Derivative liabilities	47.3	63.9
Level 2 [Member] \| Commodity Contracts [Member]
Derivative assets	7.7	10.0
Derivative liabilities	47.3	63.9
IPL [Member] \| Level 3 [Member]
Derivative assets	13.8	9.3
Foreign exchange contracts		4.8
Deferred proceeds	87.4	152.9
Available-for-sale securities	0.1	0.1
IPL [Member] \| Level 3 [Member] \| Commodity Contracts [Member]
Derivative assets	13.8	4.5
Derivative liabilities	1.4	0.2
WPL [Member] \| Level 3 [Member]
Derivative liabilities	3.7	3.2
Foreign exchange contracts		0.1
WPL [Member] \| Level 3 [Member] \| Commodity Contracts [Member]
Derivative assets	3.2	1.6
Derivative liabilities	3.7	3.1
Level 3 [Member]
Derivative assets	17.4	10.9
Foreign exchange contracts	0.4	4.8
Deferred proceeds	87.4	152.9
Available-for-sale securities	0.1	0.1
Derivative liabilities	5.1	3.4
Foreign exchange contracts		0.1
Level 3 [Member] \| Commodity Contracts [Member]
Derivative assets	17.0	6.1
Derivative liabilities	5.1	3.3
IPL [Member] \| Commodity Contracts [Member]
Derivative assets	16.7	8.1
Derivative liabilities	20.7	24.0
WPL [Member] \| Commodity Contracts [Member]
Derivative assets	8.0	8.0
Derivative liabilities	31.7	43.2
Commodity Contracts [Member]
Derivative assets	24.7	16.1
Derivative liabilities	52.4	67.2
IPL [Member]
Money market fund investments		5.6
Derivative assets	16.7	12.9
Foreign exchange contracts		4.8
Deferred proceeds	87.4	152.9
Available-for-sale securities	1.8	2.1
WPL [Member]
Money market fund investments	18.2
Derivative liabilities	31.7	43.3
Foreign exchange contracts		$ 0.1

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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