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Fair Value Measurements (Tables) | 3 Months Ended
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Mar. 31, 2021
Statement [Line Items] |
Fair Value Measurements | Carrying amounts and related estimated fair values of other financial instruments were as follows (in millions):
Alliant Energy | March 31, 2021 |  | December 31, 2020
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Money market fund investments | $8 |  |  | $8 |  |  | $— |  |  | $— |  |  | $8 |  |  | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |
Derivatives | 28 |  |  | — |  |  | 12 |  |  | 16 |  |  | 28 |  |  | 34 |  |  | — |  |  | 5 |  |  | 29 |  |  | 34 |
Deferred proceeds | 107 |  |  | — |  |  | — |  |  | 107 |  |  | 107 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 13 |  |  | — |  |  | 13 |  |  | — |  |  | 13 |  |  | 25 |  |  | — |  |  | 25 |  |  | — |  |  | 25 |
Long-term debt (incl. current maturities) | 6,779 |  |  | — |  |  | 7,613 |  |  | 1 |  |  | 7,614 |  |  | 6,777 |  |  | — |  |  | 8,107 |  |  | 2 |  |  | 8,109 |
IPL | March 31, 2021 |  | December 31, 2020
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Money market fund investments | $8 |  |  | $8 |  |  | $— |  |  | $— |  |  | $8 |  |  | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |
Derivatives | 19 |  |  | — |  |  | 4 |  |  | 15 |  |  | 19 |  |  | 29 |  |  | — |  |  | 3 |  |  | 26 |  |  | 29 |
Deferred proceeds | 107 |  |  | — |  |  | — |  |  | 107 |  |  | 107 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 6 |  |  | — |  |  | 6 |  |  | — |  |  | 6 |  |  | 12 |  |  | — |  |  | 12 |  |  | — |  |  | 12 |
Long-term debt | 3,346 |  |  | — |  |  | 3,745 |  |  | — |  |  | 3,745 |  |  | 3,345 |  |  | — |  |  | 4,021 |  |  | — |  |  | 4,021 |
WPL | March 31, 2021 |  | December 31, 2020
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | $9 |  |  | $— |  |  | $8 |  |  | $1 |  |  | $9 |  |  | $5 |  |  | $— |  |  | $2 |  |  | $3 |  |  | $5 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 7 |  |  | — |  |  | 7 |  |  | — |  |  | 7 |  |  | 13 |  |  | — |  |  | 13 |  |  | — |  |  | 13 |
Long-term debt | 2,131 |  |  | — |  |  | 2,495 |  |  | — |  |  | 2,495 |  |  | 2,130 |  |  | — |  |  | 2,690 |  |  | — |  |  | 2,690 |
Fair Value Measurements Using Significant Unobservable Inputs | Information for Alliant Energy’s and IPL’s fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions). Such amounts for WPL were not material.
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended March 31 | 2021 |  | 2020 |  | 2021 |  | 2020
Beginning balance, January 1 | $29 |  | $21 |  | $188 |  | $188
Total net losses included in changes in net assets (realized/unrealized) | (6) |  | (3) |  | — |  | —
Settlements (a) | (7) |  | (4) |  | (81) |  | —
Ending balance, March 31 | $16 |  | $14 |  | $107 |  | $188
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31 | ($6) |  | ($3) |  | $— |  | $—
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended March 31 | 2021 |  | 2020 |  | 2021 |  | 2020
Beginning balance, January 1 | $26 |  | $18 |  | $188 |  | $188
Total net losses included in changes in net assets (realized/unrealized) | (5) |  | (2) |  | — |  | —
Settlements (a) | (6) |  | (3) |  | (81) |  | —
Ending balance, March 31 | $15 |  | $13 |  | $107 |  | $188
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31 | ($6) |  | ($2) |  | $— |  | $—
(a)Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for expected credit losses associated with the receivables sold and cash amounts received from the receivables sold.
Fair Value Of Net Derivative Assets | The fair value of FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs
March 31, 2021 | $11 |  | $5 |  | $11 |  | $4 |  | $— |  | $1
December 31, 2020 | 18 |  | 11 |  | 17 |  | 9 |  | 1 |  | 2
IPL [Member] |
Statement [Line Items] |
Fair Value Measurements | Carrying amounts and related estimated fair values of other financial instruments were as follows (in millions):
Alliant Energy | March 31, 2021 |  | December 31, 2020
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Money market fund investments | $8 |  |  | $8 |  |  | $— |  |  | $— |  |  | $8 |  |  | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |
Derivatives | 28 |  |  | — |  |  | 12 |  |  | 16 |  |  | 28 |  |  | 34 |  |  | — |  |  | 5 |  |  | 29 |  |  | 34 |
Deferred proceeds | 107 |  |  | — |  |  | — |  |  | 107 |  |  | 107 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 13 |  |  | — |  |  | 13 |  |  | — |  |  | 13 |  |  | 25 |  |  | — |  |  | 25 |  |  | — |  |  | 25 |
Long-term debt (incl. current maturities) | 6,779 |  |  | — |  |  | 7,613 |  |  | 1 |  |  | 7,614 |  |  | 6,777 |  |  | — |  |  | 8,107 |  |  | 2 |  |  | 8,109 |
IPL | March 31, 2021 |  | December 31, 2020
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Money market fund investments | $8 |  |  | $8 |  |  | $— |  |  | $— |  |  | $8 |  |  | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |
Derivatives | 19 |  |  | — |  |  | 4 |  |  | 15 |  |  | 19 |  |  | 29 |  |  | — |  |  | 3 |  |  | 26 |  |  | 29 |
Deferred proceeds | 107 |  |  | — |  |  | — |  |  | 107 |  |  | 107 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 6 |  |  | — |  |  | 6 |  |  | — |  |  | 6 |  |  | 12 |  |  | — |  |  | 12 |  |  | — |  |  | 12 |
Long-term debt | 3,346 |  |  | — |  |  | 3,745 |  |  | — |  |  | 3,745 |  |  | 3,345 |  |  | — |  |  | 4,021 |  |  | — |  |  | 4,021 |
WPL | March 31, 2021 |  | December 31, 2020
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | $9 |  |  | $— |  |  | $8 |  |  | $1 |  |  | $9 |  |  | $5 |  |  | $— |  |  | $2 |  |  | $3 |  |  | $5 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 7 |  |  | — |  |  | 7 |  |  | — |  |  | 7 |  |  | 13 |  |  | — |  |  | 13 |  |  | — |  |  | 13 |
Long-term debt | 2,131 |  |  | — |  |  | 2,495 |  |  | — |  |  | 2,495 |  |  | 2,130 |  |  | — |  |  | 2,690 |  |  | — |  |  | 2,690 |
Fair Value Measurements Using Significant Unobservable Inputs | Information for Alliant Energy’s and IPL’s fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions). Such amounts for WPL were not material.
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended March 31 | 2021 |  | 2020 |  | 2021 |  | 2020
Beginning balance, January 1 | $29 |  | $21 |  | $188 |  | $188
Total net losses included in changes in net assets (realized/unrealized) | (6) |  | (3) |  | — |  | —
Settlements (a) | (7) |  | (4) |  | (81) |  | —
Ending balance, March 31 | $16 |  | $14 |  | $107 |  | $188
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31 | ($6) |  | ($3) |  | $— |  | $—
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended March 31 | 2021 |  | 2020 |  | 2021 |  | 2020
Beginning balance, January 1 | $26 |  | $18 |  | $188 |  | $188
Total net losses included in changes in net assets (realized/unrealized) | (5) |  | (2) |  | — |  | —
Settlements (a) | (6) |  | (3) |  | (81) |  | —
Ending balance, March 31 | $15 |  | $13 |  | $107 |  | $188
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at March 31 | ($6) |  | ($2) |  | $— |  | $—
(a)Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for expected credit losses associated with the receivables sold and cash amounts received from the receivables sold.
Fair Value Of Net Derivative Assets | The fair value of FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs
March 31, 2021 | $11 |  | $5 |  | $11 |  | $4 |  | $— |  | $1
December 31, 2020 | 18 |  | 11 |  | 17 |  | 9 |  | 1 |  | 2
WPL [Member] |
Statement [Line Items] |
Fair Value Measurements | Carrying amounts and related estimated fair values of other financial instruments were as follows (in millions):
Alliant Energy | March 31, 2021 |  | December 31, 2020
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Money market fund investments | $8 |  |  | $8 |  |  | $— |  |  | $— |  |  | $8 |  |  | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |
Derivatives | 28 |  |  | — |  |  | 12 |  |  | 16 |  |  | 28 |  |  | 34 |  |  | — |  |  | 5 |  |  | 29 |  |  | 34 |
Deferred proceeds | 107 |  |  | — |  |  | — |  |  | 107 |  |  | 107 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 13 |  |  | — |  |  | 13 |  |  | — |  |  | 13 |  |  | 25 |  |  | — |  |  | 25 |  |  | — |  |  | 25 |
Long-term debt (incl. current maturities) | 6,779 |  |  | — |  |  | 7,613 |  |  | 1 |  |  | 7,614 |  |  | 6,777 |  |  | — |  |  | 8,107 |  |  | 2 |  |  | 8,109 |
IPL | March 31, 2021 |  | December 31, 2020
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Money market fund investments | $8 |  |  | $8 |  |  | $— |  |  | $— |  |  | $8 |  |  | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |
Derivatives | 19 |  |  | — |  |  | 4 |  |  | 15 |  |  | 19 |  |  | 29 |  |  | — |  |  | 3 |  |  | 26 |  |  | 29 |
Deferred proceeds | 107 |  |  | — |  |  | — |  |  | 107 |  |  | 107 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 6 |  |  | — |  |  | 6 |  |  | — |  |  | 6 |  |  | 12 |  |  | — |  |  | 12 |  |  | — |  |  | 12 |
Long-term debt | 3,346 |  |  | — |  |  | 3,745 |  |  | — |  |  | 3,745 |  |  | 3,345 |  |  | — |  |  | 4,021 |  |  | — |  |  | 4,021 |
WPL | March 31, 2021 |  | December 31, 2020
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | $9 |  |  | $— |  |  | $8 |  |  | $1 |  |  | $9 |  |  | $5 |  |  | $— |  |  | $2 |  |  | $3 |  |  | $5 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 7 |  |  | — |  |  | 7 |  |  | — |  |  | 7 |  |  | 13 |  |  | — |  |  | 13 |  |  | — |  |  | 13 |
Long-term debt | 2,131 |  |  | — |  |  | 2,495 |  |  | — |  |  | 2,495 |  |  | 2,130 |  |  | — |  |  | 2,690 |  |  | — |  |  | 2,690 |
Fair Value Of Net Derivative Assets | The fair value of FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs
March 31, 2021 | $11 |  | $5 |  | $11 |  | $4 |  | $— |  | $1
December 31, 2020 | 18 |  | 11 |  | 17 |  | 9 |  | 1 |  | 2

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