Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 217.5	$ 124.4
Average amount outstanding (based on daily outstanding balances)	99.8	27.7
Weighted average interest rates	0.40%	0.30%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	35.4	54.4
Average amount outstanding (based on daily outstanding balances)	5.9	6.0
Weighted average interest rates	0.40%	0.30%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	86.6	96.5
Average amount outstanding (based on daily outstanding balances)	$ 11.7	$ 17.6
Weighted average interest rates	0.30%	0.30%