XML 159 R127.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative assets | $ (32.7) | $ 17.9 | $ 4.4
Excluding FTRs [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative liabilities | (43.1) | (7.0) |
FTRs [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative assets | 10.4 | 24.9 |
IPL [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative assets | (1.9) | 19.4 | 14.6
IPL [Member] | Excluding FTRs [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative liabilities | (12.3) | (3.2) |
IPL [Member] | FTRs [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative assets | 10.4 | 22.6 |
WPL [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative assets | (30.8) | (1.5) | $ (10.2)
WPL [Member] | Excluding FTRs [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative liabilities | (30.8) | (3.8) |
WPL [Member] | FTRs [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative assets | $ 0.0 | $ 2.3 |

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_FairValueByAssetClassAxis=us-gaap_CommodityContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_EnergyAxis=lnt_ExcludingFinancialTransmissionRightsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_EnergyAxis=lnt_FinancialTransmissionRightsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |