Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 424.4	$ 251.8
Average amount outstanding (based on daily outstanding balances)	$ 294.3	$ 179.0
Weighted average interest rates	1.20%	0.60%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 20.0	$ 3.1
Average amount outstanding (based on daily outstanding balances)	$ 0.5	$ 0.0
Weighted average interest rates	1.30%	0.70%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 271.2	$ 118.3
Average amount outstanding (based on daily outstanding balances)	$ 118.2	$ 38.1
Weighted average interest rates	1.00%	0.40%