XML 114 R38.htm IDEA: XBRL DOCUMENT
v2.4.0.8

Regulatory Matters (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 6 Months Ended |  | 6 Months Ended |  | 6 Months Ended |  | 12 Months Ended | 6 Months Ended | 12 Months Ended | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended | 1 Months Ended |  |  | 1 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2014 | Jun. 30, 2013 | Dec. 31, 2013 | Jun. 30, 2014 WPL [Member] | Jun. 30, 2013 WPL [Member] | Dec. 31, 2013 WPL [Member] | Jun. 30, 2014 IPL [Member] | Jun. 30, 2013 IPL [Member] | Dec. 31, 2013 IPL [Member] | Dec. 31, 2013 IPL [Member] Revenue Requirement Adjustment [Member] | Jun. 30, 2014 Alliant Energy and IPL [Member] Tax benefit riders [Member] | Dec. 31, 2014 Scenario, Forecast [Member] IPL [Member] Revenue Requirement Adjustment [Member] | Jul. 31, 2014 2015/2016 Test Period Retail Electric And Gas [Member] Subsequent Event [Member] WPL [Member] | Jun. 30, 2014 2013 Test Year Retail Electric [Member] IPL [Member] | Jun. 30, 2014 2013 Test Year Retail Electric [Member] IPL [Member] | Dec. 31, 2016 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member] | Dec. 31, 2015 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member] | Dec. 31, 2014 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member] | Jun. 30, 2014 2015 Test Year Retail Electric Fuel Related Request [Member] WPL [Member] | Dec. 31, 2015 2015 Test Year Retail Electric Fuel Related Request [Member] Scenario, Forecast [Member] WPL [Member] | Dec. 31, 2014 2015 Test Year Retail Electric Fuel Related Request [Member] Scenario, Forecast [Member] WPL [Member] | Dec. 31, 2013 2014 Test Year Retail Electric Fuel Related Request [Member] WPL [Member] | Jun. 30, 2014 2014 Test Year Retail Electric Fuel Related Request [Member] Alliant Energy and WPL [Member]
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities | $ 863.5 |  | $ 821.5 | $ 254.9 |  | $ 206.6 | $ 608.6 |  | $ 614.9 |  | $ 26.0 |  |  |  |  |  |  |  |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  |  |  |  |  |  |  |  |  |  |  |  | (5) |  |  |  |  |  |  |  |  | 19 |
Customer billing credits |  |  |  |  |  |  |  |  |  |  |  |  |  | 20 | 20 | 10 | 25 | 70 |  |  |  |  |
Increase (decrease) in regulatory liabilities | 45.2 | (15.1) |  | 47.9 | 12.0 |  | (2.7) | (27.1) |  | (24.0) | (48.0) | (15.0) |  |  |  |  |  |  |  |  |  |  |
Requested increase (decrease) in rates, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55 |  |  |  |
Requested increase (decrease) in rates, percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.00% |  |  |  |
Under-collection of fuel-related costs outside of approved range |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 41 | $ 14 |  | $ 17
Authorized increase (decrease) in final rates, percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |
Annual bandwidth for fuel-related costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |

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 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

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

X
- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
Name: | us-gaap_RegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant