Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 248.0	$ 181.2	$ 248.0	$ 181.2
Average amount outstanding (based on daily outstanding balances)	$ 220.1	$ 122.4	$ 210.7	$ 114.5
Weighted average interest rates	0.60%	0.40%	0.60%	0.40%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 3.1	$ 18.4	$ 3.1	$ 18.4
Average amount outstanding (based on daily outstanding balances)	$ 0.1	$ 0.5	$ 0.0	$ 0.2
Weighted average interest rates	0.60%	0.40%	0.60%	0.40%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 55.4	$ 0.0	$ 62.9	$ 0.0
Average amount outstanding (based on daily outstanding balances)	$ 36.4	$ 0.0	$ 33.2	$ 0.0
Weighted average interest rates	0.40%		0.40%