Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 353.8	$ 284.1	$ 353.8	$ 284.1
Average amount outstanding (based on daily outstanding balances)	307.1	228.8	281.9	197.2
Weighted average interest rates	0.20%	0.20%	0.20%	0.20%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	38.0	0	38.0	26.3
Average amount outstanding (based on daily outstanding balances)	0.4	0	0.3	1.7
Weighted average interest rates	0.30%		0.20%	0.40%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	185.0	165.4	204.7	165.4
Average amount outstanding (based on daily outstanding balances)	$ 157.9	$ 140.9	$ 157.5	$ 113.4
Weighted average interest rates	0.10%	0.20%	0.10%	0.20%