Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 185.3	$ 22.1	$ 185.3	$ 96.5
Average amount outstanding (based on daily outstanding balances)	133.0	2.6	100.2	28.7
Weighted average interest rates	0.40%	0.40%	0.40%	0.30%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	19.5	18.0	35.4	54.4
Average amount outstanding (based on daily outstanding balances)	1.3	1.7	7.2	7.1
Weighted average interest rates	0.40%	0.30%	0.40%	0.30%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	35.6	0	35.6	96.5
Average amount outstanding (based on daily outstanding balances)	$ 11.9	$ 0	$ 13.3	$ 23.0
Weighted average interest rates	0.30%		0.30%	0.30%