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Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Commodity Contracts [Member] |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | $ 11.9 |  | $ (0.9) |
Total net gains (losses) (realized/unrealized) included in changes in net assets | (2.4) | [1] | (12.5) | [1]
Transfers into Level 3 | 0 | [2] | (3.8) | [2]
Transfers out of Level 3 | 3.6 | [3] | 5.3 | [3]
Settlements | (8.2) | [4] | (3.2) | [4]
Ending balance | 4.9 |  | (15.1) |
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 March 31 | (2.4) | [1] | (12.5) | [1]
Commodity Contracts [Member] | IPL [Member] |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 12.5 |  | 4.3 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | (2.9) | [1] | (10.8) | [1]
Transfers into Level 3 | 0 | [2] | (2.7) | [2]
Transfers out of Level 3 | 1.1 | [3] | 0.1 | [3]
Settlements | (6.4) | [4] | (2.3) | [4]
Ending balance | 4.3 |  | (11.4) |
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 March 31 | (2.9) | [1] | (10.8) | [1]
Commodity Contracts [Member] | WPL [Member] |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | (0.6) |  | (5.2) |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0.5 | [1] | (1.7) | [1]
Transfers into Level 3 | 0 | [2] | (1.1) | [2]
Transfers out of Level 3 | 2.5 | [3] | 5.2 | [3]
Settlements | (1.8) |  | (0.9) |
Ending balance | 0.6 |  | (3.7) |
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 March 31 | 0.5 | [1] | (1.7) | [1]
Deferred Proceeds [Member] |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 66.8 |  | 53.7 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0 | [1] | 0 | [1]
Transfers into Level 3 | 0 | [2] | 0 | [2]
Transfers out of Level 3 | 0 | [3] | 0 | [3]
Settlements | 55.3 | [4] | (20.8) | [4]
Ending balance | 122.1 |  | 32.9 |
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 March 31 | 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 | 66.8 |  | 53.7 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0 | [1] | 0 | [1]
Transfers into Level 3 | 0 | [2] | 0 | [2]
Transfers out of Level 3 | 0 | [3] | 0 | [3]
Settlements | 55.3 | [4] | (20.8) | [4]
Ending balance | 122.1 |  | 32.9 |
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 March 31 | $ 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.

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- 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=7578670&loc=d3e19207-110258 + Details
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- Definition Amount of gain (loss) recognized in earnings, arising from assets measured at fair value on a recurring basis using unobservable inputs (level 3). + 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)(1) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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- 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=7578670&loc=d3e19207-110258 + Details
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- 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=7578670&loc=d3e19207-110258 + Details
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- 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=7578670&loc=d3e19207-110258 + Details
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- 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 Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 Reference 2: 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. + Details
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