Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 151.3	$ 316.2
Average amount outstanding (based on daily outstanding balances)	127.9	275.6
Weighted average interest rates	0.40%	0.10%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	1.4	10.0
Average amount outstanding (based on daily outstanding balances)	0	0.3
Weighted average interest rates	0.40%	0.20%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	0	204.7
Average amount outstanding (based on daily outstanding balances)	$ 0	$ 173.0
Weighted average interest rates		0.10%