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Regulatory Matters (Tables)
 12 Months Ended
Dec. 31, 2013
Regulatory Assets [Line Items]  
Schedule of Regulatory Assets
At December 31, regulatory assets were comprised of the following items (in millions):
 
Alliant Energy
 
IPL
 
WPL
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
Tax-related

$829.7

 

$770.7

 

$798.6

 

$746.2

 

$31.1

 

$24.5

Pension and other postretirement benefits costs
355.3

 
549.2

 
174.2

 
279.3

 
181.1

 
269.9

AROs
65.7

 
62.4

 
36.7

 
38.6

 
29.0

 
23.8

Emission allowances
30.0

 
30.0

 
30.0

 
30.0

 
—

 
—

Environmental-related costs
25.0

 
34.9

 
21.0

 
30.3

 
4.0

 
4.6

Derivatives
21.1

 
40.2

 
5.9

 
16.3

 
15.2

 
23.9

Debt redemption costs
17.9

 
19.8

 
12.2

 
13.6

 
5.7

 
6.2

IPL’s electric transmission service costs
8.3

 
16.6

 
8.3

 
16.6

 
—

 
—

Other
60.2

 
88.6

 
26.6

 
47.0

 
33.6

 
41.6

 

$1,413.2

 

$1,612.4

 

$1,113.5

 

$1,217.9

 

$299.7

 

$394.5
Schedule of Regulatory Liabilities
At December 31, regulatory liabilities were comprised of the following items (in millions):
 
Alliant Energy
 
IPL
 
WPL
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
Cost of removal obligations

$418.9

 

$408.7

 

$277.7

 

$268.0

 

$141.2

 

$140.7

IPL’s tax benefit riders
265.4

 
355.8

 
265.4

 
355.8

 
—

 
—

Energy conservation cost recovery
52.7

 
55.1

 
9.3

 
10.0

 
43.4

 
45.1

IPL’s electric transmission assets sale
21.6

 
32.5

 
21.6

 
32.5

 
—

 
—

IPL’s electric transmission cost recovery
14.6

 
—

 
14.6

 
—

 
—

 
—

Commodity cost recovery
7.5

 
17.7

 
5.5

 
5.2

 
2.0

 
12.5

Other
40.8

 
46.3

 
20.8

 
29.9

 
20.0

 
16.4

 

$821.5

 

$916.1

 

$614.9

 

$701.4

 

$206.6

 

$214.7
IPL [Member]
  
Regulatory Assets [Line Items]  
Schedule of Regulatory Assets
At December 31, regulatory assets were comprised of the following items (in millions):
 
Alliant Energy
 
IPL
 
WPL
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
Tax-related

$829.7

 

$770.7

 

$798.6

 

$746.2

 

$31.1

 

$24.5

Pension and other postretirement benefits costs
355.3

 
549.2

 
174.2

 
279.3

 
181.1

 
269.9

AROs
65.7

 
62.4

 
36.7

 
38.6

 
29.0

 
23.8

Emission allowances
30.0

 
30.0

 
30.0

 
30.0

 
—

 
—

Environmental-related costs
25.0

 
34.9

 
21.0

 
30.3

 
4.0

 
4.6

Derivatives
21.1

 
40.2

 
5.9

 
16.3

 
15.2

 
23.9

Debt redemption costs
17.9

 
19.8

 
12.2

 
13.6

 
5.7

 
6.2

IPL’s electric transmission service costs
8.3

 
16.6

 
8.3

 
16.6

 
—

 
—

Other
60.2

 
88.6

 
26.6

 
47.0

 
33.6

 
41.6

 

$1,413.2

 

$1,612.4

 

$1,113.5

 

$1,217.9

 

$299.7

 

$394.5
Schedule of Regulatory Liabilities
At December 31, regulatory liabilities were comprised of the following items (in millions):
 
Alliant Energy
 
IPL
 
WPL
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
Cost of removal obligations

$418.9

 

$408.7

 

$277.7

 

$268.0

 

$141.2

 

$140.7

IPL’s tax benefit riders
265.4

 
355.8

 
265.4

 
355.8

 
—

 
—

Energy conservation cost recovery
52.7

 
55.1

 
9.3

 
10.0

 
43.4

 
45.1

IPL’s electric transmission assets sale
21.6

 
32.5

 
21.6

 
32.5

 
—

 
—

IPL’s electric transmission cost recovery
14.6

 
—

 
14.6

 
—

 
—

 
—

Commodity cost recovery
7.5

 
17.7

 
5.5

 
5.2

 
2.0

 
12.5

Other
40.8

 
46.3

 
20.8

 
29.9

 
20.0

 
16.4

 

$821.5

 

$916.1

 

$614.9

 

$701.4

 

$206.6

 

$214.7
WPL [Member]
  
Regulatory Assets [Line Items]  
Schedule of Regulatory Assets
At December 31, regulatory assets were comprised of the following items (in millions):
 
Alliant Energy
 
IPL
 
WPL
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
Tax-related

$829.7

 

$770.7

 

$798.6

 

$746.2

 

$31.1

 

$24.5

Pension and other postretirement benefits costs
355.3

 
549.2

 
174.2

 
279.3

 
181.1

 
269.9

AROs
65.7

 
62.4

 
36.7

 
38.6

 
29.0

 
23.8

Emission allowances
30.0

 
30.0

 
30.0

 
30.0

 
—

 
—

Environmental-related costs
25.0

 
34.9

 
21.0

 
30.3

 
4.0

 
4.6

Derivatives
21.1

 
40.2

 
5.9

 
16.3

 
15.2

 
23.9

Debt redemption costs
17.9

 
19.8

 
12.2

 
13.6

 
5.7

 
6.2

IPL’s electric transmission service costs
8.3

 
16.6

 
8.3

 
16.6

 
—

 
—

Other
60.2

 
88.6

 
26.6

 
47.0

 
33.6

 
41.6

 

$1,413.2

 

$1,612.4

 

$1,113.5

 

$1,217.9

 

$299.7

 

$394.5
Schedule of Regulatory Liabilities
At December 31, regulatory liabilities were comprised of the following items (in millions):
 
Alliant Energy
 
IPL
 
WPL
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
Cost of removal obligations

$418.9

 

$408.7

 

$277.7

 

$268.0

 

$141.2

 

$140.7

IPL’s tax benefit riders
265.4

 
355.8

 
265.4

 
355.8

 
—

 
—

Energy conservation cost recovery
52.7

 
55.1

 
9.3

 
10.0

 
43.4

 
45.1

IPL’s electric transmission assets sale
21.6

 
32.5

 
21.6

 
32.5

 
—

 
—

IPL’s electric transmission cost recovery
14.6

 
—

 
14.6

 
—

 
—

 
—

Commodity cost recovery
7.5

 
17.7

 
5.5

 
5.2

 
2.0

 
12.5

Other
40.8

 
46.3

 
20.8

 
29.9

 
20.0

 
16.4

 

$821.5

 

$916.1

 

$614.9

 

$701.4

 

$206.6

 

$214.7