Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 316.2	$ 243.4
Average amount outstanding (based on daily outstanding balances)	275.6	170.8
Weighted average interest rates	0.10%	0.30%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	10.0	26.3
Average amount outstanding (based on daily outstanding balances)	0.3	4.8
Weighted average interest rates	0.20%	0.40%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	204.7	160.0
Average amount outstanding (based on daily outstanding balances)	$ 173.0	$ 72.9
Weighted average interest rates	0.10%	0.20%