Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 462.5	$ 600.6
Average amount outstanding (based on daily outstanding balances)	$ 382.3	$ 498.8
Weighted average interest rates	1.80%	2.70%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 0.8	$ 50.4
Average amount outstanding (based on daily outstanding balances)	$ 0.0	$ 0.6
Weighted average interest rates	1.80%	2.80%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 212.0	$ 195.1
Average amount outstanding (based on daily outstanding balances)	$ 167.0	$ 138.1
Weighted average interest rates	1.80%	2.50%