Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 181.2	$ 353.8	$ 181.2	$ 353.8
Average amount outstanding (based on daily outstanding balances)	$ 122.4	$ 307.1	$ 114.5	$ 281.9
Weighted average interest rates	0.40%	0.20%	0.40%	0.20%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 18.4	$ 38.0	$ 18.4	$ 38.0
Average amount outstanding (based on daily outstanding balances)	$ 0.5	$ 0.4	$ 0.2	$ 0.3
Weighted average interest rates	0.40%	0.30%	0.40%	0.20%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 0.0	$ 185.0	$ 0.0	$ 204.7
Average amount outstanding (based on daily outstanding balances)	$ 0.0	$ 157.9	$ 0.0	$ 157.5
Weighted average interest rates		0.10%		0.10%