|
Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $)
In Millions, unless otherwise specified
|
Sep. 30, 2012
|
Dec. 31, 2011
|
|
Alliant Energy Corporation [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
$ 13.6 |
$ 0 |
| Deferred proceeds (sales of receivables) |
153.9 |
53.7 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
3,598.7 |
|
| Cumulative preferred stock of subsidiaries |
215.6 |
|
|
IPL [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
1.1 |
0 |
| Deferred proceeds (sales of receivables) |
153.9 |
53.7 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
1,696.1 |
|
| Cumulative preferred stock |
154.8 |
|
|
WPL [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
12.5 |
0 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
1,492.4 |
|
| Cumulative preferred stock |
60.8 |
|
|
Commodity Contracts [Member] | Alliant Energy Corporation [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
46.5 |
15.7 |
| Capitalization and liabilities: |
|
|
| Derivatives |
57.3 |
78.0 |
|
Commodity Contracts [Member] | IPL [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
28.1 |
10.6 |
| Capitalization and liabilities: |
|
|
| Derivatives |
22.8 |
33.6 |
|
Commodity Contracts [Member] | WPL [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
18.4 |
5.1 |
| Capitalization and liabilities: |
|
|
| Derivatives |
34.5 |
44.4 |
|
Level 1 [Member] | Alliant Energy Corporation [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
13.6 |
0 |
| Deferred proceeds (sales of receivables) |
0 |
0 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
0 |
|
| Cumulative preferred stock of subsidiaries |
165.2 |
|
|
Level 1 [Member] | IPL [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
1.1 |
0 |
| Deferred proceeds (sales of receivables) |
0 |
0 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
0 |
|
| Cumulative preferred stock |
154.8 |
|
|
Level 1 [Member] | WPL [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
12.5 |
0 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
0 |
|
| Cumulative preferred stock |
10.4 |
|
|
Level 1 [Member] | Commodity Contracts [Member] | Alliant Energy Corporation [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
0 |
0 |
| Capitalization and liabilities: |
|
|
| Derivatives |
0 |
0 |
|
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
0 |
0 |
| Capitalization and liabilities: |
|
|
| Derivatives |
0 |
0 |
|
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
0 |
0 |
| Capitalization and liabilities: |
|
|
| Derivatives |
0 |
0 |
|
Level 2 [Member] | Alliant Energy Corporation [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
0 |
0 |
| Deferred proceeds (sales of receivables) |
0 |
0 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
3,598.2 |
|
| Cumulative preferred stock of subsidiaries |
50.4 |
|
|
Level 2 [Member] | IPL [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
0 |
0 |
| Deferred proceeds (sales of receivables) |
0 |
0 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
1,696.1 |
|
| Cumulative preferred stock |
0 |
|
|
Level 2 [Member] | WPL [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
0 |
0 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
1,492.4 |
|
| Cumulative preferred stock |
50.4 |
|
|
Level 2 [Member] | Commodity Contracts [Member] | Alliant Energy Corporation [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
11.9 |
3.4 |
| Capitalization and liabilities: |
|
|
| Derivatives |
40.3 |
64.8 |
|
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
8.1 |
1.3 |
| Capitalization and liabilities: |
|
|
| Derivatives |
18.3 |
28.6 |
|
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
3.8 |
2.1 |
| Capitalization and liabilities: |
|
|
| Derivatives |
22.0 |
36.2 |
|
Level 3 [Member] | Alliant Energy Corporation [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
0 |
0 |
| Deferred proceeds (sales of receivables) |
153.9 |
53.7 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
0.5 |
|
| Cumulative preferred stock of subsidiaries |
0 |
|
|
Level 3 [Member] | IPL [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
0 |
0 |
| Deferred proceeds (sales of receivables) |
153.9 |
53.7 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
0 |
|
| Cumulative preferred stock |
0 |
|
|
Level 3 [Member] | WPL [Member]
|
|
|
| Assets: |
|
|
| Money market funds, at carrying value |
0 |
0 |
| Capitalization and liabilities: |
|
|
| Long-term debt (including current maturities) |
0 |
|
| Cumulative preferred stock |
0 |
|
|
Level 3 [Member] | Commodity Contracts [Member] | Alliant Energy Corporation [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
34.6 |
12.3 |
| Capitalization and liabilities: |
|
|
| Derivatives |
17.0 |
13.2 |
|
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
20.0 |
9.3 |
| Capitalization and liabilities: |
|
|
| Derivatives |
4.5 |
5.0 |
|
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
|
|
|
| Assets: |
|
|
| Derivative assets |
14.6 |
3.0 |
| Capitalization and liabilities: |
|
|
| Derivatives |
$ 12.5 |
$ 8.2 |