Regulatory Matters (Customer Billing Credits) (Details) - USD ($) $ in Millions	12 Months Ended			36 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
IPL [Member]
Regulatory Liabilities [Line Items]
Billing credits to reduce retail electric customers' bills	$ 9	$ 24	$ 72	$ 105