Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 446.5	$ 424.4
Average amount outstanding (based on daily outstanding balances)	$ 221.4	$ 294.3
Weighted average interest rates	2.20%	1.20%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 50.4	$ 20.0
Average amount outstanding (based on daily outstanding balances)	$ 1.5	$ 0.5
Weighted average interest rates	2.30%	1.30%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 126.0	$ 271.2
Average amount outstanding (based on daily outstanding balances)	$ 36.6	$ 118.2
Weighted average interest rates	2.10%	1.00%