Benefit Plans (Estimated Future And Actual Contributions) (Details) (USD $) In Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2011
Estimated Future and Actual Employer Contributions		[1]		[1]
Defined Benefit Pension Plans [Member]
Estimated Future and Actual Employer Contributions	9		10
IPL [Member] | Other Postretirement Benefit Plans [Member]
Estimated Future and Actual Employer Contributions	2		3
WPL [Member] | Other Postretirement Benefit Plans [Member]
Estimated Future and Actual Employer Contributions	4		5
Other Postretirement Benefit Plans [Member]
Estimated Future and Actual Employer Contributions	7		9
Non-Qualified Defined Benefit Pension Plans [Member]
Estimated Future and Actual Employer Contributions	$ 6		$ 7

[1] Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans.