Common Equity (Narrative) (Details) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
IPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	$ 375,000,000
Restricted net assets of subsidiaries	1,100,000,000
Capital contributions from parent	80,000,000	0
Common stock dividends to Alliant Energy	91,600,000
WPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	209,000,000
Restricted net assets of subsidiaries	1,300,000,000
Capital contributions from parent	0	25,000,000
Common stock dividends to Alliant Energy	$ 84,100,000