Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 243.4	$ 102.8
Average amount outstanding (based on daily outstanding balances)	170.8	66.4
Weighted average interest rates	0.30%	0.30%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	26.3	35.4
Average amount outstanding (based on daily outstanding balances)	4.8	12.8
Weighted average interest rates	0.40%	0.40%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	160.0	32.7
Average amount outstanding (based on daily outstanding balances)	$ 72.9	$ 13.1
Weighted average interest rates	0.20%	0.20%