Regulatory Matters (Customer Billing Credits) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
IPL [Member]
Regulatory Liabilities [Line Items]
Customer billing credits	$ 2	$ 6	$ 4	$ 12

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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Period Type:	duration