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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2018
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
 
2018
 
2017
 
2018
 
2017
 
2018
 
2017
Balance, January 1

$184.5

 

$195.7

 

$134.1

 

$124.7

 

$50.4

 

$61.4

Revisions in estimated cash flows
(10.1
)
 
4.3

 
(10.1
)
 
7.0

 
—

 
(2.7
)
Liabilities settled
(10.4
)
 
(23.5
)
 
(9.7
)
 
(13.1
)
 
(0.7
)
 
(10.4
)
Liabilities incurred
7.3

 
2.0

 
—

 
11.7

 
7.3

 
—

Accretion expense
6.2

 
6.0

 
4.0

 
3.8

 
2.2

 
2.1

Balance, December 31

$177.5

 

$184.5

 

$118.3

 

$134.1

 

$59.2

 

$50.4
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
 
2018
 
2017
 
2018
 
2017
 
2018
 
2017
Balance, January 1

$184.5

 

$195.7

 

$134.1

 

$124.7

 

$50.4

 

$61.4

Revisions in estimated cash flows
(10.1
)
 
4.3

 
(10.1
)
 
7.0

 
—

 
(2.7
)
Liabilities settled
(10.4
)
 
(23.5
)
 
(9.7
)
 
(13.1
)
 
(0.7
)
 
(10.4
)
Liabilities incurred
7.3

 
2.0

 
—

 
11.7

 
7.3

 
—

Accretion expense
6.2

 
6.0

 
4.0

 
3.8

 
2.2

 
2.1

Balance, December 31

$177.5

 

$184.5

 

$118.3

 

$134.1

 

$59.2

 

$50.4
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
 
2018
 
2017
 
2018
 
2017
 
2018
 
2017
Balance, January 1

$184.5

 

$195.7

 

$134.1

 

$124.7

 

$50.4

 

$61.4

Revisions in estimated cash flows
(10.1
)
 
4.3

 
(10.1
)
 
7.0

 
—

 
(2.7
)
Liabilities settled
(10.4
)
 
(23.5
)
 
(9.7
)
 
(13.1
)
 
(0.7
)
 
(10.4
)
Liabilities incurred
7.3

 
2.0

 
—

 
11.7

 
7.3

 
—

Accretion expense
6.2

 
6.0

 
4.0

 
3.8

 
2.2

 
2.1

Balance, December 31

$177.5

 

$184.5

 

$118.3

 

$134.1

 

$59.2

 

$50.4