Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 223.1	$ 162.8	$ 243.4	$ 162.8
Average amount outstanding (based on daily outstanding balances)	191.4	100.8	181.2	83.6
Weighted average interest rates	0.20%	0.40%	0.30%	0.40%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	6.0	30.3	26.3	35.4
Average amount outstanding (based on daily outstanding balances)	0.2	7.6	2.5	10.2
Weighted average interest rates	0.30%	0.40%	0.40%	0.40%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	159.4	35.6	160.0	35.6
Average amount outstanding (based on daily outstanding balances)	$ 125.7	$ 14.9	$ 99.4	$ 14.0
Weighted average interest rates	0.20%	0.30%	0.20%	0.20%