Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 311.6	$ 223.1	$ 316.2	$ 243.4
Average amount outstanding (based on daily outstanding balances)	262.7	191.4	269.1	181.2
Weighted average interest rates	0.20%	0.20%	0.20%	0.30%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	6.0	6.0	10.0	26.3
Average amount outstanding (based on daily outstanding balances)	0.1	0.2	0.2	2.5
Weighted average interest rates	0.20%	0.30%	0.20%	0.40%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	168.4	159.4	204.7	160.0
Average amount outstanding (based on daily outstanding balances)	$ 141.8	$ 125.7	$ 157.3	$ 99.4
Weighted average interest rates	0.10%	0.20%	0.10%	0.20%