Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 229.9	$ 152.6	$ 242.6	$ 152.6
Average amount outstanding (based on daily outstanding balances)	$ 213.0	$ 93.2	$ 206.0	$ 110.5
Weighted average interest rates	0.60%	0.50%	0.60%	0.40%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 1.7	$ 9.7	$ 1.7	$ 9.7
Average amount outstanding (based on daily outstanding balances)	$ 0.0	$ 0.1	$ 0.0	$ 0.1
Weighted average interest rates	0.60%	0.50%	0.60%	0.50%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 62.9	$ 0.0	$ 62.9	$ 0.0
Average amount outstanding (based on daily outstanding balances)	$ 37.4	$ 0.0	$ 31.6	$ 0.0
Weighted average interest rates	0.40%		0.40%