v2.4.0.6

Benefit Plans (Estimated Future Funding) (Details) (Scenario, Forecast [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2012
--- | ---
Qualified Defined Benefit Pension Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2013 | $ 0 |
Qualified Defined Benefit Pension Plans [Member] | IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2013 | 0 |
Qualified Defined Benefit Pension Plans [Member] | WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2013 | 0 |
Non-qualified defined benefit pension plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2013 | 2.4 | [1]
Directly assigned defined benefit pension plans [Member] | IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2013 | 0.8 | [2]
Directly assigned defined benefit pension plans [Member] | WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2013 | 0.2 | [2]
Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2013 | 3.0 |
Other Postretirement Benefits Plans [Member] | IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2013 | 0 |
Other Postretirement Benefits Plans [Member] | WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2013 | $ 3.0 |
[1] | Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans.
[2] | Amounts directly assigned to IPL and WPL for non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.

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

X
- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph g -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. + Details
Name: | us-gaap_DefinedBenefitPlanEstimatedFutureEmployerContributionsInNextFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant