Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Maximum amount outstanding (based on daily outstanding balances)	$ 102.8	$ 96.5
Average amount outstanding (based on daily outstanding balances)	66.4	55.4
Weighted average interest rates	0.30%	0.30%
IPL [Member]
Maximum amount outstanding (based on daily outstanding balances)	35.4	12.1
Average amount outstanding (based on daily outstanding balances)	12.8	1.2
Weighted average interest rates	0.40%	0.30%
WPL [Member]
Maximum amount outstanding (based on daily outstanding balances)	32.7	96.5
Average amount outstanding (based on daily outstanding balances)	$ 13.1	$ 55.4
Weighted average interest rates	0.20%	0.30%