Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 424.4	$ 248.0	$ 424.4	$ 248.0
Average amount outstanding (based on daily outstanding balances)	$ 386.2	$ 220.1	$ 323.9	$ 210.7
Weighted average interest rates	1.30%	0.60%	1.10%	0.60%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 20.0	$ 3.1	$ 20.0	$ 3.1
Average amount outstanding (based on daily outstanding balances)	$ 0.4	$ 0.1	$ 0.5	$ 0.0
Weighted average interest rates	1.40%	0.60%	1.20%	0.60%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 271.2	$ 55.4	$ 271.2	$ 62.9
Average amount outstanding (based on daily outstanding balances)	$ 217.0	$ 36.4	$ 144.2	$ 33.2
Weighted average interest rates	1.10%	0.40%	1.00%	0.40%