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Fair Value Measurements (Tables) | 12 Months Ended
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Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |
Recurring Fair Value Measurements | Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions):
Alliant Energy | 2020 |  | 2019
|  |  | 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 | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |  |  | $5 |  |  | $5 |  |  | $— |  |  | $— |  |  | $5 |
Derivatives | 34 |  |  | — |  |  | 5 |  |  | 29 |  |  | 34 |  |  | 27 |  |  | — |  |  | 5 |  |  | 22 |  |  | 27 |
Deferred proceeds | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 25 |  |  | — |  |  | 25 |  |  | — |  |  | 25 |  |  | 38 |  |  | — |  |  | 37 |  |  | 1 |  |  | 38 |
Long-term debt (incl. current maturities) | 6,777 |  |  | — |  |  | 8,107 |  |  | 2 |  |  | 8,109 |  |  | 6,190 |  |  | — |  |  | 6,918 |  |  | 2 |  |  | 6,920 |
IPL | 2020 |  | 2019
|  |  | 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 | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |  |  | $5 |  |  | $5 |  |  | $— |  |  | $— |  |  | $5 |
Derivatives | 29 |  |  | — |  |  | 3 |  |  | 26 |  |  | 29 |  |  | 22 |  |  | — |  |  | 3 |  |  | 19 |  |  | 22 |
Deferred proceeds | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 12 |  |  | — |  |  | 12 |  |  | — |  |  | 12 |  |  | 18 |  |  | — |  |  | 17 |  |  | 1 |  |  | 18 |
Long-term debt (incl. current maturities) | 3,345 |  |  | — |  |  | 4,021 |  |  | — |  |  | 4,021 |  |  | 3,147 |  |  | — |  |  | 3,489 |  |  | — |  |  | 3,489 |
WPL | 2020 |  | 2019
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | $5 |  |  | $— |  |  | $2 |  |  | $3 |  |  | $5 |  |  | $5 |  |  | $— |  |  | $2 |  |  | $3 |  |  | $5 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 13 |  |  | — |  |  | 13 |  |  | — |  |  | 13 |  |  | 20 |  |  | — |  |  | 20 |  |  | — |  |  | 20 |
Long-term debt (incl. current maturities) | 2,130 |  |  | — |  |  | 2,690 |  |  | — |  |  | 2,690 |  |  | 1,933 |  |  | — |  |  | 2,268 |  |  | — |  |  | 2,268 |
Fair Value Measurements Using Significant Unobservable Inputs | Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions):
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
| 2020 |  | 2019 |  | 2020 |  | 2019
Beginning balance, January 1 | $21 |  |  | $12 |  |  | $188 |  |  | $119 |
Total net gains included in changes in net assets (realized/unrealized) | 11 |  |  | 8 |  |  | — |  |  | — |
Transfers out of Level 3 (a) | — |  |  | 4 |  |  | — |  |  | — |
Purchases | 14 |  |  | 14 |  |  | — |  |  | — |
Sales | (1) |  |  | — |  |  | — |  |  | — |
Settlements (b) | (16) |  |  | (17) |  |  | — |  |  | 69 |
Ending balance, December 31 | $29 |  |  | $21 |  |  | $188 |  |  | $188 |
The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 | $11 |  |  | $11 |  |  | $— |  |  | $— |
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
| 2020 |  | 2019 |  | 2020 |  | 2019
Beginning balance, January 1 | $18 |  |  | $9 |  |  | $188 |  |  | $119 |
Total net gains included in changes in net assets (realized/unrealized) | 10 |  |  | 11 |  |  | — |  |  | — |
Transfers out of Level 3 (a) | — |  |  | 2 |  |  | — |  |  | — |
Purchases | 11 |  |  | 9 |  |  | — |  |  | — |
Sales | (1) |  |  | — |  |  | — |  |  | — |
Settlements (b) | (12) |  |  | (13) |  |  | — |  |  | 69 |
Ending balance, December 31 | $26 |  |  | $18 |  |  | $188 |  |  | $188 |
The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 | $10 |  |  | $12 |  |  | $— |  |  | $— |
(a)Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3. (b)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 (Liabilities) | The fair value of FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets at December 31 as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs
2020 | $18 |  |  | $11 |  |  | $17 |  |  | $9 |  |  | $1 |  |  | $2 |
2019 | 15 |  |  | 7 |  |  | 14 |  |  | 5 |  |  | 1 |  |  | 2 |
IPL [Member] |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |
Recurring Fair Value Measurements | Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions):
Alliant Energy | 2020 |  | 2019
|  |  | 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 | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |  |  | $5 |  |  | $5 |  |  | $— |  |  | $— |  |  | $5 |
Derivatives | 34 |  |  | — |  |  | 5 |  |  | 29 |  |  | 34 |  |  | 27 |  |  | — |  |  | 5 |  |  | 22 |  |  | 27 |
Deferred proceeds | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 25 |  |  | — |  |  | 25 |  |  | — |  |  | 25 |  |  | 38 |  |  | — |  |  | 37 |  |  | 1 |  |  | 38 |
Long-term debt (incl. current maturities) | 6,777 |  |  | — |  |  | 8,107 |  |  | 2 |  |  | 8,109 |  |  | 6,190 |  |  | — |  |  | 6,918 |  |  | 2 |  |  | 6,920 |
IPL | 2020 |  | 2019
|  |  | 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 | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |  |  | $5 |  |  | $5 |  |  | $— |  |  | $— |  |  | $5 |
Derivatives | 29 |  |  | — |  |  | 3 |  |  | 26 |  |  | 29 |  |  | 22 |  |  | — |  |  | 3 |  |  | 19 |  |  | 22 |
Deferred proceeds | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 12 |  |  | — |  |  | 12 |  |  | — |  |  | 12 |  |  | 18 |  |  | — |  |  | 17 |  |  | 1 |  |  | 18 |
Long-term debt (incl. current maturities) | 3,345 |  |  | — |  |  | 4,021 |  |  | — |  |  | 4,021 |  |  | 3,147 |  |  | — |  |  | 3,489 |  |  | — |  |  | 3,489 |
WPL | 2020 |  | 2019
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | $5 |  |  | $— |  |  | $2 |  |  | $3 |  |  | $5 |  |  | $5 |  |  | $— |  |  | $2 |  |  | $3 |  |  | $5 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 13 |  |  | — |  |  | 13 |  |  | — |  |  | 13 |  |  | 20 |  |  | — |  |  | 20 |  |  | — |  |  | 20 |
Long-term debt (incl. current maturities) | 2,130 |  |  | — |  |  | 2,690 |  |  | — |  |  | 2,690 |  |  | 1,933 |  |  | — |  |  | 2,268 |  |  | — |  |  | 2,268 |
Fair Value Measurements Using Significant Unobservable Inputs | Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions):
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
| 2020 |  | 2019 |  | 2020 |  | 2019
Beginning balance, January 1 | $21 |  |  | $12 |  |  | $188 |  |  | $119 |
Total net gains included in changes in net assets (realized/unrealized) | 11 |  |  | 8 |  |  | — |  |  | — |
Transfers out of Level 3 (a) | — |  |  | 4 |  |  | — |  |  | — |
Purchases | 14 |  |  | 14 |  |  | — |  |  | — |
Sales | (1) |  |  | — |  |  | — |  |  | — |
Settlements (b) | (16) |  |  | (17) |  |  | — |  |  | 69 |
Ending balance, December 31 | $29 |  |  | $21 |  |  | $188 |  |  | $188 |
The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 | $11 |  |  | $11 |  |  | $— |  |  | $— |
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
| 2020 |  | 2019 |  | 2020 |  | 2019
Beginning balance, January 1 | $18 |  |  | $9 |  |  | $188 |  |  | $119 |
Total net gains included in changes in net assets (realized/unrealized) | 10 |  |  | 11 |  |  | — |  |  | — |
Transfers out of Level 3 (a) | — |  |  | 2 |  |  | — |  |  | — |
Purchases | 11 |  |  | 9 |  |  | — |  |  | — |
Sales | (1) |  |  | — |  |  | — |  |  | — |
Settlements (b) | (12) |  |  | (13) |  |  | — |  |  | 69 |
Ending balance, December 31 | $26 |  |  | $18 |  |  | $188 |  |  | $188 |
The amount of total net gains for the period included in changes in net assets attributable to the change in unrealized gains relating to assets and liabilities held at December 31 | $10 |  |  | $12 |  |  | $— |  |  | $— |
(a)Observable market inputs became available for certain commodity contracts previously classified as Level 3 for transfers out of Level 3. (b)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 (Liabilities) | The fair value of FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets at December 31 as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs
2020 | $18 |  |  | $11 |  |  | $17 |  |  | $9 |  |  | $1 |  |  | $2 |
2019 | 15 |  |  | 7 |  |  | 14 |  |  | 5 |  |  | 1 |  |  | 2 |
WPL [Member] |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |
Recurring Fair Value Measurements | Carrying amounts and the related estimated fair values of other financial instruments at December 31 were as follows (in millions):
Alliant Energy | 2020 |  | 2019
|  |  | 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 | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |  |  | $5 |  |  | $5 |  |  | $— |  |  | $— |  |  | $5 |
Derivatives | 34 |  |  | — |  |  | 5 |  |  | 29 |  |  | 34 |  |  | 27 |  |  | — |  |  | 5 |  |  | 22 |  |  | 27 |
Deferred proceeds | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 25 |  |  | — |  |  | 25 |  |  | — |  |  | 25 |  |  | 38 |  |  | — |  |  | 37 |  |  | 1 |  |  | 38 |
Long-term debt (incl. current maturities) | 6,777 |  |  | — |  |  | 8,107 |  |  | 2 |  |  | 8,109 |  |  | 6,190 |  |  | — |  |  | 6,918 |  |  | 2 |  |  | 6,920 |
IPL | 2020 |  | 2019
|  |  | 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 | $44 |  |  | $44 |  |  | $— |  |  | $— |  |  | $44 |  |  | $5 |  |  | $5 |  |  | $— |  |  | $— |  |  | $5 |
Derivatives | 29 |  |  | — |  |  | 3 |  |  | 26 |  |  | 29 |  |  | 22 |  |  | — |  |  | 3 |  |  | 19 |  |  | 22 |
Deferred proceeds | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |  |  | 188 |  |  | — |  |  | — |  |  | 188 |  |  | 188 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 12 |  |  | — |  |  | 12 |  |  | — |  |  | 12 |  |  | 18 |  |  | — |  |  | 17 |  |  | 1 |  |  | 18 |
Long-term debt (incl. current maturities) | 3,345 |  |  | — |  |  | 4,021 |  |  | — |  |  | 4,021 |  |  | 3,147 |  |  | — |  |  | 3,489 |  |  | — |  |  | 3,489 |
WPL | 2020 |  | 2019
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | $5 |  |  | $— |  |  | $2 |  |  | $3 |  |  | $5 |  |  | $5 |  |  | $— |  |  | $2 |  |  | $3 |  |  | $5 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 13 |  |  | — |  |  | 13 |  |  | — |  |  | 13 |  |  | 20 |  |  | — |  |  | 20 |  |  | — |  |  | 20 |
Long-term debt (incl. current maturities) | 2,130 |  |  | — |  |  | 2,690 |  |  | — |  |  | 2,690 |  |  | 1,933 |  |  | — |  |  | 2,268 |  |  | — |  |  | 2,268 |
Fair Value Of Net Derivative Assets (Liabilities) | The fair value of FTR and natural gas commodity contracts categorized as Level 3 was recognized as net derivative assets at December 31 as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs
2020 | $18 |  |  | $11 |  |  | $17 |  |  | $9 |  |  | $1 |  |  | $2 |
2019 | 15 |  |  | 7 |  |  | 14 |  |  | 5 |  |  | 1 |  |  | 2 |

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- Definition Tabular disclosure of the fair value measurement of assets using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes during the period attributable to the following: (1) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets) and gains or losses recognized in other comprehensive income (loss), and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (2) purchases, sales, issues, and settlements (each type disclosed separately); and (3) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs), by class of asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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