Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Money market fund investments	$ 18.2	$ 128.3
Derivative assets	25.1	20.9
Deferred proceeds of receivables sold	87.4	152.9
Available-for-sale securities	1.8	2.1
Cumulative preferred stock of subsidiaries, Carrying Value	145.1	183.8
Cumulative preferred stock of subsidiaries, Carrying Value	60.0	60.0
Derivative liabilities	52.4	67.3
IPL [Member] \| Carrying Value [Member]
Money market fund investments		5.6
Derivative assets	16.7	12.9
Deferred proceeds of receivables sold	87.4	152.9
Available-for-sale securities	1.8	2.1
Long-term debt (including current maturities), Carrying Value	1,308.9	1,308.6
Cumulative preferred stock of subsidiaries, Carrying Value	145.1	183.8
Derivative liabilities	20.7	24.0
WPL [Member] \| Carrying Value [Member]
Money market fund investments	18.2
Derivative assets	8.0	8.0
Long-term debt (including current maturities), Carrying Value	1,082.1	1,081.7
Cumulative preferred stock of subsidiaries, Carrying Value	60.0	60.0
Derivative liabilities	31.7	43.3
Carrying Value [Member]
Money market fund investments	18.2	128.3
Derivative assets	25.1	20.9
Deferred proceeds of receivables sold	87.4	152.9
Available-for-sale securities	1.8	2.1
Long-term debt (including current maturities), Carrying Value	2,705.0	2,704.7
Cumulative preferred stock of subsidiaries, Carrying Value	205.1	243.8
Derivative liabilities	52.4	67.3
IPL [Member] \| Fair Value [Member]
Money market fund investments		5.6
Derivative assets	16.7	12.9
Deferred proceeds of receivables sold	87.4	152.9
Available-for-sale securities	1.8	2.1
Long-term debt (including current maturities), Fair Value	1,563.7	1,417.0
Cumulative preferred stock of subsidiaries, Fair Value	166.5	210.7
Derivative liabilities	20.7	24.0
WPL [Member] \| Fair Value [Member]
Money market fund investments	18.2
Derivative assets	8.0	8.0
Long-term debt (including current maturities), Fair Value	1,421.3	1,219.6
Cumulative preferred stock of subsidiaries, Fair Value	56.3	56.0
Derivative liabilities	31.7	43.3
Fair Value [Member]
Money market fund investments	18.2	128.3
Derivative assets	25.1	20.9
Deferred proceeds of receivables sold	87.4	152.9
Available-for-sale securities	1.8	2.1
Long-term debt (including current maturities), Fair Value	3,315.1	2,958.6
Cumulative preferred stock of subsidiaries, Fair Value	222.8	266.7
Derivative liabilities	52.4	67.3
IPL [Member]
Money market fund investments		5.6
Derivative assets	16.7	12.9
Deferred proceeds of receivables sold	87.4	152.9
Available-for-sale securities	1.8	2.1
Cumulative preferred stock of subsidiaries, Carrying Value	145.1	183.8
WPL [Member]
Money market fund investments	18.2
Cumulative preferred stock of subsidiaries, Carrying Value	60.0	60.0
Derivative liabilities	$ 31.7	$ 43.3

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Preferred Stock Fair Value

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The carrying value (book value) of preferred stock of subsidiaries.

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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.

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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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Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date. May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.

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Aggregate par or stated value of issued nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable preferred shares, par value and other disclosure concepts are in another section within stockholders' equity.

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