Leases (Future Minimum Operating Lease Payments, Excluding Contingent Rentals) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Component of Operating Other Cost and Expense [Line Items]
2013	$ 9
2014	13
2015	5
2016	3
2017	2
Thereafter	24
Total future minimum operating lease payments, excluding contingent rentals	56
IPL [Member]
Component of Operating Other Cost and Expense [Line Items]
2013	4
2014	3
2015	3
2016	2
2017	2
Thereafter	16
Total future minimum operating lease payments, excluding contingent rentals	30
WPL [Member]
Component of Operating Other Cost and Expense [Line Items]
2013	4
2014	9
2015	2
2016	1
2017	0
Thereafter	0
Total future minimum operating lease payments, excluding contingent rentals	$ 16