Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Maximum amount outstanding (based on daily outstanding balances)	$ 124.4	$ 350.3
Average amount outstanding (based on daily outstanding balances)	27.7	92.0
Weighted average interest rates	0.30%	0.30%
IPL [Member]
Maximum amount outstanding (based on daily outstanding balances)	54.4	219.1
Average amount outstanding (based on daily outstanding balances)	6.0	62.8
Weighted average interest rates	0.30%	0.30%
WPL [Member]
Maximum amount outstanding (based on daily outstanding balances)	96.5	170.2
Average amount outstanding (based on daily outstanding balances)	$ 17.6	$ 36.6
Weighted average interest rates	0.30%	0.30%