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Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Assets: |  |
Money market fund investments | $ 44 | $ 5
Deferred proceeds | 188 | 188
Liabilities and equity: |  |
Long-term debt (including current maturities) | 8,109 | 6,920
Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 34 | 27
Liabilities and equity: |  |
Derivatives | 25 | 38
Level 1 [Member] |  |
Assets: |  |
Money market fund investments | 44 | 5
Deferred proceeds | 0 | 0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0 | 0
Level 1 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 0 | 0
Liabilities and equity: |  |
Derivatives | 0 | 0
Level 2 [Member] |  |
Assets: |  |
Money market fund investments | 0 | 0
Deferred proceeds | 0 | 0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 8,107 | 6,918
Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 5 | 5
Liabilities and equity: |  |
Derivatives | 25 | 37
Level 3 [Member] |  |
Assets: |  |
Money market fund investments | 0 | 0
Deferred proceeds | 188 | 188
Liabilities and equity: |  |
Long-term debt (including current maturities) | 2 | 2
Level 3 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 29 | 22
Liabilities and equity: |  |
Derivatives | 0 | 1
Carrying Amount [Member] |  |
Assets: |  |
Money market fund investments | 44 | 5
Deferred proceeds | 188 | 188
Liabilities and equity: |  |
Long-term debt (including current maturities) | 6,777 | 6,190
Carrying Amount [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 34 | 27
Liabilities and equity: |  |
Derivatives | 25 | 38
IPL [Member] |  |
Assets: |  |
Money market fund investments | 44 | 5
Deferred proceeds | 188 | 188
Liabilities and equity: |  |
Long-term debt (including current maturities) | 4,021 | 3,489
IPL [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 29 | 22
Liabilities and equity: |  |
Derivatives | 12 | 18
IPL [Member] | Level 1 [Member] |  |
Assets: |  |
Money market fund investments | 44 | 5
Deferred proceeds | 0 | 0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0 | 0
IPL [Member] | Level 1 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 0 | 0
Liabilities and equity: |  |
Derivatives | 0 | 0
IPL [Member] | Level 2 [Member] |  |
Assets: |  |
Money market fund investments | 0 | 0
Deferred proceeds | 0 | 0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 4,021 | 3,489
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 3 | 3
Liabilities and equity: |  |
Derivatives | 12 | 17
IPL [Member] | Level 3 [Member] |  |
Assets: |  |
Money market fund investments | 0 | 0
Deferred proceeds | 188 | 188
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0 | 0
IPL [Member] | Level 3 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 26 | 19
Liabilities and equity: |  |
Derivatives | 0 | 1
IPL [Member] | Carrying Amount [Member] |  |
Assets: |  |
Money market fund investments | 44 | 5
Deferred proceeds | 188 | 188
Liabilities and equity: |  |
Long-term debt (including current maturities) | 3,345 | 3,147
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 29 | 22
Liabilities and equity: |  |
Derivatives | 12 | 18
WPL [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 2,690 | 2,268
WPL [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 5 | 5
Liabilities and equity: |  |
Derivatives | 13 | 20
WPL [Member] | Level 1 [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0 | 0
WPL [Member] | Level 1 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 0 | 0
Liabilities and equity: |  |
Derivatives | 0 | 0
WPL [Member] | Level 2 [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 2,690 | 2,268
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 2 | 2
Liabilities and equity: |  |
Derivatives | 13 | 20
WPL [Member] | Level 3 [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0 | 0
WPL [Member] | Level 3 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 3 | 3
Liabilities and equity: |  |
Derivatives | 0 | 0
WPL [Member] | Carrying Amount [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 2,130 | 1,933
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 5 | 5
Liabilities and equity: |  |
Derivatives | $ 13 | $ 20

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