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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2021
Schedule of Asset Retirement Obligations [Line Items]  
Reconciliation of Changes in Asset Retirement Obligations
A reconciliation of the changes in asset retirement obligations associated with long-lived assets for the six months ended June 30, 2021 is as follows (in millions):
Alliant EnergyIPL
Balance, January 1$251$177
Revisions in estimated cash flows3226
Liabilities settled(4)(3)
Accretion expense43
Balance, June 30
$283$203
IPL [Member]  
Schedule of Asset Retirement Obligations [Line Items]  
Reconciliation of Changes in Asset Retirement Obligations
A reconciliation of the changes in asset retirement obligations associated with long-lived assets for the six months ended June 30, 2021 is as follows (in millions):
Alliant EnergyIPL
Balance, January 1$251$177
Revisions in estimated cash flows3226
Liabilities settled(4)(3)
Accretion expense43
Balance, June 30
$283$203