Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 181.2	$ 353.8
Average amount outstanding (based on daily outstanding balances)	$ 119.2	$ 255.9
Weighted average interest rates	0.40%	0.20%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 18.4	$ 38.0
Average amount outstanding (based on daily outstanding balances)	$ 0.2	$ 0.2
Weighted average interest rates	0.40%	0.20%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 24.7	$ 204.7
Average amount outstanding (based on daily outstanding balances)	$ 2.2	$ 122.9
Weighted average interest rates	0.30%	0.10%