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Related Parties (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
WPL Owed ATC [Member]
Related Party Transaction [Line Items]
Net amounts owed	$ 7	$ 6
IPL [Member]
Related Party Transaction [Line Items]
Net amounts owed	79	72
WPL [Member]
Related Party Transaction [Line Items]
Net amounts owed	$ 42	$ 40

Receivables to be collected from (obligations owed to) related parties, net as of the balance sheet date where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth.

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