Common Equity (Narrative) (Details) (USD $)	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
IPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	$ 360,000,000
Restricted net assets of subsidiaries	1,000,000,000
Capital contributions from parent	50,000,000	0
Common stock dividends to Alliant Energy	60,800,000
WPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	105,000,000
Restricted net assets of subsidiaries	1,300,000,000
Capital contributions from parent	0	25,000,000
Common stock dividends to Alliant Energy	$ 56,000,000