Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 293.9	$ 217.5
Average amount outstanding (based on daily outstanding balances)	210.5	99.8
Weighted average interest rates	0.20%	0.40%
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)	1.3	5.9
Weighted average interest rates	0.40%	0.40%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	190.0	86.6
Average amount outstanding (based on daily outstanding balances)	$ 123.5	$ 11.7
Weighted average interest rates	0.20%	0.30%