XML 42 R37.htm IDEA: XBRL DOCUMENT
v2.4.0.8

Regulatory Matters (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 1 Months Ended | 3 Months Ended | 12 Months Ended | 3 Months Ended |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2014 | Mar. 31, 2013 | Mar. 31, 2014 WPL [Member] | Mar. 31, 2013 WPL [Member] | Dec. 31, 2013 WPL [Member] 2014 Test Year Retail Electric Fuel Related Request [Member] | Mar. 31, 2014 IPL [Member] | Mar. 31, 2013 IPL [Member] | Dec. 31, 2013 IPL [Member] Revenue Requirement Adjustment [Member] | Mar. 31, 2014 Alliant Energy and IPL [Member] Revenue Requirement Adjustment [Member] | Mar. 31, 2013 Alliant Energy and IPL [Member] Revenue Requirement Adjustment [Member] | Mar. 31, 2014 Alliant Energy and WPL [Member] 2014 Test Year Retail Electric Fuel Related Request [Member] | Apr. 30, 2014 Subsequent Event [Member] WPL [Member] 2015/2016 Test Period Retail Electric And Gas [Member] | Dec. 31, 2014 Scenario, Forecast [Member] IPL [Member] Revenue Requirement Adjustment [Member] | Dec. 31, 2016 Scenario, Forecast [Member] IPL [Member] 2013 Test Year Retail Electric [Member] | Dec. 31, 2015 Scenario, Forecast [Member] IPL [Member] 2013 Test Year Retail Electric [Member] | Dec. 31, 2014 Scenario, Forecast [Member] IPL [Member] 2013 Test Year Retail Electric [Member]
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate decrease, amount |  |  |  |  |  |  |  |  |  |  |  | $ 5 |  |  |  |
Customer billing credits |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 | 25 | 70
Increase (decrease) in regulatory liabilities |  |  | 26.1 | 5.8 |  | (9.0) | 14.7 | (24.0) |  |  |  |  | (15.0) |  |  |
Increase to electric revenues | 675.8 | 633.2 | 301.6 | 283.0 |  | 374.2 | 350.2 |  | 3.9 | 5.5 |  |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  |  |  |  | 19 |  |  |  |  |  |  |  |  |  |  |
Authorized increase (decrease) in final rates, percent |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |
Annual bandwidth for fuel-related costs |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |
Under-collection of fuel-related costs outside of approved range |  |  |  |  |  |  |  |  |  |  | $ 10 |  |  |  |  |

X
- Definition Customer Billing Credits + References No definition available. + Details
Name: | lnt_CustomerBillingCredits
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Rate of fuel costs capacity + References No definition available. + Details
Name: | lnt_RateOfFuelCostsCapacity
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Carrying amount as of the balance sheet date of capitalized fuel costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378 + Details
Name: | us-gaap_DeferredFuelCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Revenue derived from the regulated (by a federal, state, or local government or agency) generation, transmission and distribution of electricity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(b)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_ElectricDomesticRegulatedRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
Name: | us-gaap_IncreaseDecreaseInRegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of public utility's requested rate increase (decrease) with regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration