Regulatory Matters (Customer Billing Credits) (Details) (IPL [Member], USD $) In Millions, unless otherwise specified	12 Months Ended			36 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Billing credits to reduce retail electric customers' bills	$ 72
Scenario, Forecast [Member]
Regulatory Liabilities [Line Items]
Billing credits to reduce retail electric customers' bills		10	25	105
Target [Member]
Regulatory Liabilities [Line Items]
Billing credits to reduce retail electric customers' bills	$ 70