Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 153.3	$ 424.4	$ 446.5	$ 424.4
Average amount outstanding (based on daily outstanding balances)	$ 80.8	$ 386.2	$ 207.6	$ 323.9
Weighted average interest rates	2.20%	1.30%	2.10%	1.10%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 0.0	$ 20.0	$ 31.4	$ 20.0
Average amount outstanding (based on daily outstanding balances)	0.0	$ 0.4	$ 1.7	$ 0.5
Weighted average interest rates		1.40%	2.30%	1.20%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	75.4	$ 271.2	$ 109.4	$ 271.2
Average amount outstanding (based on daily outstanding balances)	$ 23.1	$ 217.0	$ 26.6	$ 144.2
Weighted average interest rates	2.00%	1.10%	1.90%	1.00%