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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2019
Schedule of Asset Retirement Obligations [Line Items]  
Reconciliation Of Changes In Asset Retirement Obligations (AROs) A reconciliation of the changes in AROs associated with long-lived assets is as follows (in millions):
 
Alliant Energy
 
IPL
 
WPL
 
2019
 
2018
 
2019
 
2018
 
2019
 
2018
Balance, January 1

$177.5

 

$184.5

 

$118.3

 

$134.1

 

$59.2

 

$50.4

Revisions in estimated cash flows
(5.5
)
 
(10.1
)
 
(6.7
)
 
(10.1
)
 
1.2

 
—

Liabilities settled
(8.8
)
 
(10.4
)
 
(8.4
)
 
(9.7
)
 
(0.4
)
 
(0.7
)
Liabilities incurred (a)
26.2

 
7.3

 
26.2

 
—

 
—

 
7.3

Accretion expense
6.9

 
6.2

 
4.5

 
4.0

 
2.4

 
2.2

Balance, December 31

$196.3

 

$177.5

 

$133.9

 

$118.3

 

$62.4

 

$59.2


(a)
In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.
IPL [Member]  
Schedule of Asset Retirement Obligations [Line Items]  
Reconciliation Of Changes In Asset Retirement Obligations (AROs) A reconciliation of the changes in AROs associated with long-lived assets is as follows (in millions):
 
Alliant Energy
 
IPL
 
WPL
 
2019
 
2018
 
2019
 
2018
 
2019
 
2018
Balance, January 1

$177.5

 

$184.5

 

$118.3

 

$134.1

 

$59.2

 

$50.4

Revisions in estimated cash flows
(5.5
)
 
(10.1
)
 
(6.7
)
 
(10.1
)
 
1.2

 
—

Liabilities settled
(8.8
)
 
(10.4
)
 
(8.4
)
 
(9.7
)
 
(0.4
)
 
(0.7
)
Liabilities incurred (a)
26.2

 
7.3

 
26.2

 
—

 
—

 
7.3

Accretion expense
6.9

 
6.2

 
4.5

 
4.0

 
2.4

 
2.2

Balance, December 31

$196.3

 

$177.5

 

$133.9

 

$118.3

 

$62.4

 

$59.2


(a)
In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.
WPL [Member]  
Schedule of Asset Retirement Obligations [Line Items]  
Reconciliation Of Changes In Asset Retirement Obligations (AROs) A reconciliation of the changes in AROs associated with long-lived assets is as follows (in millions):
 
Alliant Energy
 
IPL
 
WPL
 
2019
 
2018
 
2019
 
2018
 
2019
 
2018
Balance, January 1

$177.5

 

$184.5

 

$118.3

 

$134.1

 

$59.2

 

$50.4

Revisions in estimated cash flows
(5.5
)
 
(10.1
)
 
(6.7
)
 
(10.1
)
 
1.2

 
—

Liabilities settled
(8.8
)
 
(10.4
)
 
(8.4
)
 
(9.7
)
 
(0.4
)
 
(0.7
)
Liabilities incurred (a)
26.2

 
7.3

 
26.2

 
—

 
—

 
7.3

Accretion expense
6.9

 
6.2

 
4.5

 
4.0

 
2.4

 
2.2

Balance, December 31

$196.3

 

$177.5

 

$133.9

 

$118.3

 

$62.4

 

$59.2


(a)
In 2019, Alliant Energy and IPL recognized additional AROs related to IPL’s newly constructed Upland Prairie and English Farms wind sites. The increases in AROs resulted in corresponding increases in property, plant and equipment, net on the respective balance sheets.