Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 270.5	$ 590.2	$ 462.5	$ 600.6
Average amount outstanding (based on daily outstanding balances)	$ 121.2	$ 531.7	$ 251.8	$ 515.4
Weighted average interest rates	0.50%	2.70%	1.50%	2.70%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 8.4	$ 0.0	$ 8.4	$ 50.4
Average amount outstanding (based on daily outstanding balances)	$ 0.4	0.0	$ 0.2	$ 0.3
Weighted average interest rates	0.50%		0.50%	2.80%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 182.0	193.3	$ 212.0	$ 195.1
Average amount outstanding (based on daily outstanding balances)	$ 8.7	$ 141.6	$ 87.9	$ 139.8
Weighted average interest rates	0.50%	2.50%	1.70%	2.50%