Related Parties (Amounts Billed Between Parties) (Details) - ATC [Member] - WPL [Member] - Equity Method Investment [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
ATC Billings To WPL [Member]
Related Party Transactions [Line Items]
Amounts billed between related parties	$ 28	$ 25	$ 82	$ 75
WPL Billings To ATC [Member]
Related Party Transactions [Line Items]
Amounts billed between related parties	$ 4	$ 4	$ 10	$ 9