Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 600.6	$ 446.5
Average amount outstanding (based on daily outstanding balances)	$ 453.5	$ 221.4
Weighted average interest rates	2.50%	2.20%
IPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 50.4	$ 50.4
Average amount outstanding (based on daily outstanding balances)	$ 0.1	$ 1.5
Weighted average interest rates	2.80%	2.30%
WPL [Member]
Debt Instrument [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 195.1	$ 126.0
Average amount outstanding (based on daily outstanding balances)	$ 92.6	$ 36.6
Weighted average interest rates	2.40%	2.10%