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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
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
 
2017
 
2016
 
2017
 
2016
 
2017
 
2016
Balance, January 1

$195.7

 

$214.0

 

$124.7

 

$132.9

 

$61.4

 

$71.9

Revisions in estimated cash flows
4.3

 
(13.3
)
 
7.0

 
(5.8
)
 
(2.7
)
 
(7.5
)
Liabilities settled
(23.5
)
 
(14.0
)
 
(13.1
)
 
(6.8
)
 
(10.4
)
 
(7.2
)
Liabilities incurred
2.0

 
2.6

 
11.7

 
0.7

 
—

 
1.9

Accretion expense
6.0

 
6.4

 
3.8

 
3.7

 
2.1

 
2.3

Balance, December 31

$184.5

 

$195.7

 

$134.1

 

$124.7

 

$50.4

 

$61.4
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
 
2017
 
2016
 
2017
 
2016
 
2017
 
2016
Balance, January 1

$195.7

 

$214.0

 

$124.7

 

$132.9

 

$61.4

 

$71.9

Revisions in estimated cash flows
4.3

 
(13.3
)
 
7.0

 
(5.8
)
 
(2.7
)
 
(7.5
)
Liabilities settled
(23.5
)
 
(14.0
)
 
(13.1
)
 
(6.8
)
 
(10.4
)
 
(7.2
)
Liabilities incurred
2.0

 
2.6

 
11.7

 
0.7

 
—

 
1.9

Accretion expense
6.0

 
6.4

 
3.8

 
3.7

 
2.1

 
2.3

Balance, December 31

$184.5

 

$195.7

 

$134.1

 

$124.7

 

$50.4

 

$61.4
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
 
2017
 
2016
 
2017
 
2016
 
2017
 
2016
Balance, January 1

$195.7

 

$214.0

 

$124.7

 

$132.9

 

$61.4

 

$71.9

Revisions in estimated cash flows
4.3

 
(13.3
)
 
7.0

 
(5.8
)
 
(2.7
)
 
(7.5
)
Liabilities settled
(23.5
)
 
(14.0
)
 
(13.1
)
 
(6.8
)
 
(10.4
)
 
(7.2
)
Liabilities incurred
2.0

 
2.6

 
11.7

 
0.7

 
—

 
1.9

Accretion expense
6.0

 
6.4

 
3.8

 
3.7

 
2.1

 
2.3

Balance, December 31

$184.5

 

$195.7

 

$134.1

 

$124.7

 

$50.4

 

$61.4