v2.4.0.6

Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011
--- | --- | ---
Alliant Energy Corporation [Member] |  |
Assets: |  |
Money market funds, at carrying value | $ 13.6 | $ 0
Deferred proceeds (sales of receivables) | 153.9 | 53.7
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,598.7 |
Cumulative preferred stock of subsidiaries | 215.6 |
IPL [Member] |  |
Assets: |  |
Money market funds, at carrying value | 1.1 | 0
Deferred proceeds (sales of receivables) | 153.9 | 53.7
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,696.1 |
Cumulative preferred stock | 154.8 |
WPL [Member] |  |
Assets: |  |
Money market funds, at carrying value | 12.5 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,492.4 |
Cumulative preferred stock | 60.8 |
Commodity Contracts [Member] | Alliant Energy Corporation [Member] |  |
Assets: |  |
Derivative assets | 46.5 | 15.7
Capitalization and liabilities: |  |
Derivatives | 57.3 | 78.0
Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 28.1 | 10.6
Capitalization and liabilities: |  |
Derivatives | 22.8 | 33.6
Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 18.4 | 5.1
Capitalization and liabilities: |  |
Derivatives | 34.5 | 44.4
Level 1 [Member] | Alliant Energy Corporation [Member] |  |
Assets: |  |
Money market funds, at carrying value | 13.6 | 0
Deferred proceeds (sales of receivables) | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 |
Cumulative preferred stock of subsidiaries | 165.2 |
Level 1 [Member] | IPL [Member] |  |
Assets: |  |
Money market funds, at carrying value | 1.1 | 0
Deferred proceeds (sales of receivables) | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 |
Cumulative preferred stock | 154.8 |
Level 1 [Member] | WPL [Member] |  |
Assets: |  |
Money market funds, at carrying value | 12.5 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 |
Cumulative preferred stock | 10.4 |
Level 1 [Member] | Commodity Contracts [Member] | Alliant Energy Corporation [Member] |  |
Assets: |  |
Derivative assets | 0 | 0
Capitalization and liabilities: |  |
Derivatives | 0 | 0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 0 | 0
Capitalization and liabilities: |  |
Derivatives | 0 | 0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 0 | 0
Capitalization and liabilities: |  |
Derivatives | 0 | 0
Level 2 [Member] | Alliant Energy Corporation [Member] |  |
Assets: |  |
Money market funds, at carrying value | 0 | 0
Deferred proceeds (sales of receivables) | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,598.2 |
Cumulative preferred stock of subsidiaries | 50.4 |
Level 2 [Member] | IPL [Member] |  |
Assets: |  |
Money market funds, at carrying value | 0 | 0
Deferred proceeds (sales of receivables) | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,696.1 |
Cumulative preferred stock | 0 |
Level 2 [Member] | WPL [Member] |  |
Assets: |  |
Money market funds, at carrying value | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,492.4 |
Cumulative preferred stock | 50.4 |
Level 2 [Member] | Commodity Contracts [Member] | Alliant Energy Corporation [Member] |  |
Assets: |  |
Derivative assets | 11.9 | 3.4
Capitalization and liabilities: |  |
Derivatives | 40.3 | 64.8
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 8.1 | 1.3
Capitalization and liabilities: |  |
Derivatives | 18.3 | 28.6
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 3.8 | 2.1
Capitalization and liabilities: |  |
Derivatives | 22.0 | 36.2
Level 3 [Member] | Alliant Energy Corporation [Member] |  |
Assets: |  |
Money market funds, at carrying value | 0 | 0
Deferred proceeds (sales of receivables) | 153.9 | 53.7
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0.5 |
Cumulative preferred stock of subsidiaries | 0 |
Level 3 [Member] | IPL [Member] |  |
Assets: |  |
Money market funds, at carrying value | 0 | 0
Deferred proceeds (sales of receivables) | 153.9 | 53.7
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 |
Cumulative preferred stock | 0 |
Level 3 [Member] | WPL [Member] |  |
Assets: |  |
Money market funds, at carrying value | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 |
Cumulative preferred stock | 0 |
Level 3 [Member] | Commodity Contracts [Member] | Alliant Energy Corporation [Member] |  |
Assets: |  |
Derivative assets | 34.6 | 12.3
Capitalization and liabilities: |  |
Derivatives | 17.0 | 13.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 20.0 | 9.3
Capitalization and liabilities: |  |
Derivatives | 4.5 | 5.0
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 14.6 | 3.0
Capitalization and liabilities: |  |
Derivatives | $ 12.5 | $ 8.2

X
- Details
Name: | lnt_CapitalizationAndLiabilitiesAbstract
Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Fair value of deferred proceeds of receivables sold + References No definition available. + Details
Name: | lnt_FairValueOfDeferredProceedsOfReceivablesSold
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Preferred Stock Fair Value + References No definition available. + Details
Name: | lnt_PreferredStockFairValue
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Cumulative preferred stock of subsidiaries, fair value. + References No definition available. + Details
Name: | lnt_TemporaryEquityAndPreferredStockFairValue
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Temporary Equity Fair Value + References No definition available. + Details
Name: | lnt_TemporaryEquityFairValue
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_AssetsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13476-108611 + Details
Name: | us-gaap_DebtInstrumentFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13495-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FIN39-1 -Paragraph 10A, 10B -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 4, 17 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13433-108611 + Details
Name: | us-gaap_DerivativeAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13495-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FIN39-1 -Paragraph 10A, 10B -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 4, 17 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13433-108611 + Details
Name: | us-gaap_DerivativeLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 8, 9 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + Details
Name: | us-gaap_MoneyMarketFundsAtCarryingValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant