Short-Term Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Maximum amount of total short-term debt outstanding (based on daily outstanding balances)	$ 22.1		$ 96.5	$ 350.3
Average amount of total short-term debt outstanding (based on daily outstanding balances)	2.6		28.7	118.2
Weighted average interest rates - total short-term debt	0.37%		0.31%	0.32%
IPL [Member]
Maximum amount of total short-term debt outstanding (based on daily outstanding balances)	18.0	11.3	54.4	219.1
Average amount of total short-term debt outstanding (based on daily outstanding balances)	1.7	0.3	7.1	83.2
Weighted average interest rates - total short-term debt	0.34%	0.27%	0.32%	0.34%
WPL [Member]
Maximum amount of total short-term debt outstanding (based on daily outstanding balances)			96.5	170.2
Average amount of total short-term debt outstanding (based on daily outstanding balances)			$ 23.0	$ 44.1
Weighted average interest rates - total short-term debt	0.00%	0.00%	0.31%	0.29%