Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 162.8	$ 94.3	$ 162.8	$ 96.5
Average amount outstanding (based on daily outstanding balances)	100.8	28.7	83.6	42.0
Weighted average interest rates	0.40%	0.30%	0.40%	0.30%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	30.3	54.4	35.4	54.4
Average amount outstanding (based on daily outstanding balances)	7.6	18.3	10.2	9.8
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	44.3	35.6	96.5
Average amount outstanding (based on daily outstanding balances)	$ 14.9	$ 14.2	$ 14.0	$ 34.7
Weighted average interest rates	0.30%	0.30%	0.20%	0.30%