Regulatory Matters (Customer Billing Credits) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
IPL [Member]
Regulatory Liabilities [Line Items]
Billings credits to reduce retail electric customers' bills	$ 3	$ 7	$ 7	$ 19

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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