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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2014
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
 
2014
 
2013
 
2014
 
2013
 
2014
 
2013
Balance, January 1

$109.7

 

$101.5

 

$47.9

 

$45.5

 

$52.4

 

$46.9

Revisions in estimated cash flows
—

 
5.6

 
—

 
0.1

 
—

 
5.5

Liabilities settled
(3.4
)
 
(2.3
)
 
(1.4
)
 
(0.6
)
 
(2.0
)
 
(1.7
)
Liabilities incurred
3.7

 
1.2

 
3.5

 
1.2

 
0.2

 
—

Accretion expense
4.0

 
3.7

 
1.8

 
1.7

 
1.8

 
1.7

Balance, December 31

$114.0

 

$109.7

 

$51.8

 

$47.9

 

$52.4

 

$52.4
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
 
2014
 
2013
 
2014
 
2013
 
2014
 
2013
Balance, January 1

$109.7

 

$101.5

 

$47.9

 

$45.5

 

$52.4

 

$46.9

Revisions in estimated cash flows
—

 
5.6

 
—

 
0.1

 
—

 
5.5

Liabilities settled
(3.4
)
 
(2.3
)
 
(1.4
)
 
(0.6
)
 
(2.0
)
 
(1.7
)
Liabilities incurred
3.7

 
1.2

 
3.5

 
1.2

 
0.2

 
—

Accretion expense
4.0

 
3.7

 
1.8

 
1.7

 
1.8

 
1.7

Balance, December 31

$114.0

 

$109.7

 

$51.8

 

$47.9

 

$52.4

 

$52.4
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
 
2014
 
2013
 
2014
 
2013
 
2014
 
2013
Balance, January 1

$109.7

 

$101.5

 

$47.9

 

$45.5

 

$52.4

 

$46.9

Revisions in estimated cash flows
—

 
5.6

 
—

 
0.1

 
—

 
5.5

Liabilities settled
(3.4
)
 
(2.3
)
 
(1.4
)
 
(0.6
)
 
(2.0
)
 
(1.7
)
Liabilities incurred
3.7

 
1.2

 
3.5

 
1.2

 
0.2

 
—

Accretion expense
4.0

 
3.7

 
1.8

 
1.7

 
1.8

 
1.7

Balance, December 31

$114.0

 

$109.7

 

$51.8

 

$47.9

 

$52.4

 

$52.4