Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 590.2	$ 446.5	$ 600.6	$ 446.5
Average amount outstanding (based on daily outstanding balances)	$ 531.7	$ 234.5	$ 515.4	$ 272.1
Weighted average interest rates	2.70%	2.20%	2.70%	2.00%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 0.0	$ 31.4	$ 50.4	$ 31.4
Average amount outstanding (based on daily outstanding balances)	0.0	$ 5.1	$ 0.3	$ 2.6
Weighted average interest rates		2.30%	2.80%	2.30%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	193.3	$ 109.4	$ 195.1	$ 109.4
Average amount outstanding (based on daily outstanding balances)	$ 141.6	$ 45.2	$ 139.8	$ 28.4
Weighted average interest rates	2.50%	1.80%	2.50%	1.80%