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Benefit Plans (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Benefit Plans [Line Items] |  |
Total plan assets (less than 1%), percentage of common stock | 1.00% | 1.00%
Common stock percentage in assets held in 401(k) saving plans | 9.00% | 11.00%
Unrecognized compensation cost | $ 5 |
Alliant Energy common stock closing price on December 31, 2020 (in dollars per share) | $ 51.53 |
Percentage of base salary and short-term cash incentive compensation | 100.00% |
Carrying value of deferred compensation obligations | $ 22 | $ 21
Performance Shares Equity Awards [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Performance Shares and Performance Units Liability Awards [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Instrument valuation based on shares of common stock, number of shares | 1 |
Restricted Stock Units Equity Awards [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Instrument valuation based on shares of common stock, number of shares | 1 |
Restricted Stock Units Liability Awards [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Instrument valuation based on shares of common stock, number of shares | 1 |
Performance Restricted Stock Units Equity Awards [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Instrument valuation based on shares of common stock, number of shares | 1 |
Minimum [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost recognized over a weighted average period | 1 year |
Minimum [Member] | Performance Shares Equity Awards [Member] |  |
Benefit Plans [Line Items] |  |
Actual number of shares paid out upon vesting, percentage of target shares | 0.00% |
Minimum [Member] | Performance Shares and Performance Units Liability Awards [Member] |  |
Benefit Plans [Line Items] |  |
Actual number of shares paid out upon vesting, percentage of target shares | 0.00% |
Minimum [Member] | Performance Restricted Stock Units Equity Awards [Member] |  |
Benefit Plans [Line Items] |  |
Actual number of shares paid out upon vesting, percentage of target shares | 0.00% |
Maximum [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost recognized over a weighted average period | 2 years |
Maximum [Member] | Performance Shares Equity Awards [Member] |  |
Benefit Plans [Line Items] |  |
Actual number of shares paid out upon vesting, percentage of target shares | 200.00% |
Maximum [Member] | Performance Shares and Performance Units Liability Awards [Member] |  |
Benefit Plans [Line Items] |  |
Actual number of shares paid out upon vesting, percentage of target shares | 200.00% |
Maximum [Member] | Performance Restricted Stock Units Equity Awards [Member] |  |
Benefit Plans [Line Items] |  |
Actual number of shares paid out upon vesting, percentage of target shares | 200.00% |
IPL [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost | $ 3 |
WPL [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost | 2 |
Other Postretirement Benefits Plans [Member] |  |
Benefit Plans [Line Items] |  |
Plan asset threshold for long-term allocation targets | 5 |
Other Postretirement Benefits Plans [Member] | IPL [Member] |  |
Benefit Plans [Line Items] |  |
Plan asset threshold for long-term allocation targets | 5 |
Other Postretirement Benefits Plans [Member] | WPL [Member] |  |
Benefit Plans [Line Items] |  |
Plan asset threshold for long-term allocation targets | $ 5 |
Omnibus Incentive Plan [Member] |  |
Benefit Plans [Line Items] |  |
Shares available for issuance under the Amended and Restated OIP (in shares) | 9,000,000 |
Shares included in diluted earnings per share (in shares) | 537,485 |

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