Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 284.1	$ 185.3	$ 284.1	$ 185.3
Average amount outstanding (based on daily outstanding balances)	228.8	133.0	197.2	100.2
Weighted average interest rates	0.20%	0.40%	0.20%	0.40%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	0	19.5	26.3	35.4
Average amount outstanding (based on daily outstanding balances)	0	1.3	1.7	7.2
Weighted average interest rates		0.40%	0.40%	0.40%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	165.4	35.6	165.4	35.6
Average amount outstanding (based on daily outstanding balances)	$ 140.9	$ 11.9	$ 113.4	$ 13.3
Weighted average interest rates	0.20%	0.30%	0.20%	0.30%