Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 251.8	$ 181.2
Average amount outstanding (based on daily outstanding balances)	$ 179.0	$ 119.2
Weighted average interest rates	0.60%	0.40%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 3.1	$ 18.4
Average amount outstanding (based on daily outstanding balances)	$ 0.0	$ 0.2
Weighted average interest rates	0.70%	0.40%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 118.3	$ 24.7
Average amount outstanding (based on daily outstanding balances)	$ 38.1	$ 2.2
Weighted average interest rates	0.40%	0.30%