Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 600.6	$ 336.4
Average amount outstanding (based on daily outstanding balances)	$ 498.8	$ 310.1
Weighted average interest rates	2.70%	1.90%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 50.4	$ 3.0
Average amount outstanding (based on daily outstanding balances)	$ 0.6	$ 0.0
Weighted average interest rates	2.80%	1.80%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 195.1	$ 36.7
Average amount outstanding (based on daily outstanding balances)	$ 138.1	$ 11.5
Weighted average interest rates	2.50%	1.60%