Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 471.7	$ 153.3	$ 600.6	$ 446.5
Average amount outstanding (based on daily outstanding balances)	$ 399.5	$ 80.8	$ 476.3	$ 207.6
Weighted average interest rates	2.50%	2.20%	2.60%	2.10%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 0.0	$ 0.0	$ 50.4	$ 31.4
Average amount outstanding (based on daily outstanding balances)	0.0	0.0	$ 0.2	$ 1.7
Weighted average interest rates			2.80%	2.30%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	89.7	75.4	$ 195.1	$ 109.4
Average amount outstanding (based on daily outstanding balances)	$ 27.0	$ 23.1	$ 101.8	$ 26.6
Weighted average interest rates	2.30%	2.00%	2.50%	1.90%