Common Equity (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2012
IPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	$ 323,000,000
Restricted net assets of subsidiaries	1,000,000,000
Common stock dividends to Alliant Energy	29,700,000
WPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	84,000,000
Restricted net assets of subsidiaries	1,400,000,000
Common stock dividends to Alliant Energy	$ 28,100,000