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Debt (Narrative) (Details) (USD $)
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Dec. 31, 2012
revolving_credit_facility
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|---|---|
| Debt Instrument [Line Items] | |
| Number of revolving credit facilities included in short-term borrowing arrangements | 3 |
| Line of credit facility, current borrowing capacity | $ 1,000,000,000 |
| Non-recourse debt and hybrid securities consolidated capital manximum limit | 15.00% |
|
Parent Company [Member]
|
|
| Debt Instrument [Line Items] | |
| Line of credit facility, current borrowing capacity | 300,000,000 |
|
IPL [Member]
|
|
| Debt Instrument [Line Items] | |
| Line of credit facility, current borrowing capacity | 300,000,000 |
|
WPL [Member]
|
|
| Debt Instrument [Line Items] | |
| Line of credit facility, current borrowing capacity | $ 400,000,000 |