Common Equity (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
IPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	$ 398,000,000
Restricted net assets of subsidiaries	1,100,000,000
Capital contributions from parent	30,000,000	0
Common stock dividends to parent	31,600,000
WPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions	90,000,000
Restricted net assets of subsidiaries	1,500,000,000
Common stock dividends to parent	$ 29,100,000