v2.4.0.6

Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Commodity Contracts [Member] |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance | $ 18.8 |  | $ 18.1 |  | $ (0.9) |  | $ 2.8 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | (2.3) | [1] | (0.1) | [1] | (8.3) | [1] | 0 | [1]
Transfers into Level 3 | (0.4) | [2] | 0 | [2] | (1.7) | [2] | 0.2 | [2]
Transfers out of Level 3 | 9.3 | [3] | 0 | [3] | 8.3 | [3] | 0 | [3]
Purchases |  |  |  |  | 35.8 |  | 21.8 |
Settlements | (7.8) | [4] | (6.1) | [4] | (15.6) | [4] | (12.9) | [4]
Ending balance | 17.6 |  | 11.9 |  | 17.6 |  | 11.9 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 | (0.7) | [1] | (0.1) | [1] | (4.4) | [1] | 0 | [1]
Commodity Contracts [Member] | IPL [Member] |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance | 14.1 |  | 17.5 |  | 4.3 |  | 4.3 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | (0.2) | [1] | (0.1) | [1] | (4.8) | [1] | 0.4 | [1]
Transfers into Level 3 |  |  |  |  | (1.1) | [2] | 0 | [2]
Transfers out of Level 3 | 7.4 | [3] | 0 | [3] | 2.4 | [3] | 0 | [3]
Purchases |  |  |  |  | 26.8 |  | 18.1 |
Sales |  |  |  |  | 0 | [5] | 0 | [5]
Settlements | (5.8) | [4] | (5.0) | [4] | (12.1) | [4] | (10.4) | [4]
Ending balance | 15.5 |  | 12.4 |  | 15.5 |  | 12.4 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 | 1.4 | [1] | (0.1) | [1] | (0.7) | [1] | 0.4 | [1]
Commodity Contracts [Member] | WPL [Member] |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance | 4.7 |  | 0.6 |  | (5.2) |  | (1.5) |
Total net gains (losses) (realized/unrealized) included in changes in net assets | (2.1) | [1] | 0 | [1] | (3.5) | [1] | (0.4) | [1]
Transfers into Level 3 | (0.4) | [2] | 0 | [2] | (0.6) | [2] | 0.2 | [2]
Transfers out of Level 3 | 1.9 | [3] | 0 | [3] | 5.9 | [3] | 0 | [3]
Purchases |  |  |  |  | 9.0 |  | 3.7 |
Settlements | (2.0) |  | (1.1) |  | (3.5) |  | (2.5) |
Ending balance | 2.1 |  | (0.5) |  | 2.1 |  | (0.5) |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 | (2.1) | [1] | 0 | [1] | (3.7) | [1] | (0.4) | [1]
Foreign Contracts [Member] |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance | 0 |  | 2.1 |  | 0 |  | 4.7 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Transfers into Level 3 | 0 | [2] | 0 | [2] | 0 | [2] | 0 | [2]
Transfers out of Level 3 | 0 | [3] | 0 | [3] | 0 | [3] | 0 | [3]
Purchases |  |  |  |  | 0 |  | 0 |
Settlements | 0 | [4] | (1.7) | [4] | 0 | [4] | (4.3) | [4]
Ending balance | 0 |  | 0.4 |  | 0 |  | 0.4 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Foreign Contracts [Member] | IPL [Member] |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance | 0 |  | 0 |  | 0 |  | 4.8 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Transfers into Level 3 |  |  |  |  | 0 | [2] | 0 | [2]
Transfers out of Level 3 | 0 | [3] | 0 | [3] | 0 | [3] | 0 | [3]
Purchases |  |  |  |  | 0 |  | 0 |
Sales |  |  |  |  | 0 | [5] | (2.1) | [5]
Settlements | 0 | [4] | 0 | [4] | 0 | [4] | (2.7) | [4]
Ending balance | 0 |  | 0 |  | 0 |  | 0 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Foreign Contracts [Member] | WPL [Member] |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance | 0 |  | 0 |  | 0 |  | (0.1) |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Transfers into Level 3 | 0 | [2] | 0 | [2] | 0 | [2] | 0 | [2]
Transfers out of Level 3 | 0 | [3] | 0 | [3] | 0 | [3] | 0 | [3]
Purchases |  |  |  |  | 0 |  | 0 |
Settlements | 0 |  | 0 |  | 0 |  | 0.1 |
Ending balance | 0 |  | 0 |  | 0 |  | 0 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Deferred Proceeds [Member] |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance | 81.7 |  | 66.4 |  | 53.7 |  | 152.9 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Transfers into Level 3 | 0 | [2] | 0 | [2] | 0 | [2] | 0 | [2]
Transfers out of Level 3 | 0 | [3] | 0 | [3] | 0 | [3] | 0 | [3]
Purchases |  |  |  |  | 0 |  | 0 |
Settlements | 72.2 | [4] | 21.0 | [4] | 100.2 | [4] | (65.5) | [4]
Ending balance | 153.9 |  | 87.4 |  | 153.9 |  | 87.4 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Deferred Proceeds [Member] | IPL [Member] |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance | 81.7 |  | 66.4 |  | 53.7 |  | 152.9 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Transfers into Level 3 |  |  |  |  | 0 | [2] | 0 | [2]
Transfers out of Level 3 | 0 | [3] | 0 | [3] | 0 | [3] | 0 | [3]
Purchases |  |  |  |  | 0 |  | 0 |
Sales |  |  |  |  | 0 | [5] | 0 | [5]
Settlements | 72.2 | [4] | 21.0 | [4] | 100.2 | [4] | (65.5) | [4]
Ending balance | 153.9 |  | 87.4 |  | 153.9 |  | 87.4 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 | $ 0 | [1] | $ 0 | [1] | $ 0 | [1] | $ 0 | [1]
[1] | Gains and losses related to derivative assets and derivative liabilities are recorded in “Regulatory assets” and “Regulatory liabilities” on the Condensed Consolidated Balance Sheets.
[2] | Markets for similar assets and liabilities became inactive and observable market inputs became unavailable for transfers into Level 3. The transfers were valued as of the beginning of the period.
[3] | Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3. The transfers were valued as of the beginning of the period.
[4] | Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash proceeds received from the receivables sold.
[5] | The foreign exchange contract was transferred from IPL to Resources in connection with the sale of wind project assets in the second quarter of 2011.

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

X
- Definition The portion (amount) of unrealized gains and losses recognized as changes in net assets during the period and attributable to alternative investments continued to be held at period end. These gains or losses are attributable to certain investments for which net asset value per share is calculated for purposes of measuring fair value and which are subject to unobservable inputs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph d -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This element represents total gains or losses for the period (realized and unrealized), arising from assets measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 157 -Paragraph 32 -Subparagraph c(1) -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 -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Purchases that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Sales that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Settlements that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Transfers into assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Transfers out of assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 157 -Paragraph 32 -Subparagraph c -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 -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant