Debt (Narrative) (Details)	Dec. 31, 2016 USD ($) revolving_credit_facility
Debt Instrument [Line Items]
Number of revolving credit facilities included in short-term borrowing arrangements (number of facilities) | revolving_credit_facility	3
Line of credit facility, current borrowing capacity	$ 1,000,000,000
Non-recourse debt and hybrid securities consolidated capital maximum limit	15.00%
Parent Company [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	$ 300,000,000
IPL [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	300,000,000
WPL [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	$ 400,000,000