|
Benefit Plans (Regulatory Assets) (Details) (USD $)
In Millions, unless otherwise specified |
Dec. 31, 2012
|
Dec. 31, 2011
|
|---|---|---|
|
IPL [Member]
|
||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Regulatory assets | $ 38.1 | $ 33.7 |
| Regulatory liabilities | 0.6 | 0.3 |
|
WPL [Member]
|
||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Regulatory assets | 25.5 | 22.3 |
| Regulatory liabilities | $ 0.4 | $ 0.2 |