|
Debt (Other Short-Term Borrowings) (Details) (USD $)
In Millions, unless otherwise specified |
12 Months Ended | |
|---|---|---|
|
Dec. 31, 2012
|
Dec. 31, 2011
|
|
| Debt Instrument [Line Items] | ||
| Maximum amount outstanding (based on daily outstanding balances) | $ 217.5 | $ 124.4 |
| Average amount outstanding (based on daily outstanding balances) | 99.8 | 27.7 |
| Weighted average interest rates | 0.40% | 0.30% |
|
IPL [Member]
|
||
| Debt Instrument [Line Items] | ||
| Maximum amount outstanding (based on daily outstanding balances) | 35.4 | 54.4 |
| Average amount outstanding (based on daily outstanding balances) | 5.9 | 6.0 |
| Weighted average interest rates | 0.40% | 0.30% |
|
WPL [Member]
|
||
| Debt Instrument [Line Items] | ||
| Maximum amount outstanding (based on daily outstanding balances) | 86.6 | 96.5 |
| Average amount outstanding (based on daily outstanding balances) | $ 11.7 | $ 17.6 |
| Weighted average interest rates | 0.30% | 0.30% |