Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 325.5	$ 242.6
Average amount outstanding (based on daily outstanding balances)	$ 276.5	$ 199.0
Weighted average interest rates	0.90%	0.60%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 9.2	$ 0.0
Average amount outstanding (based on daily outstanding balances)	$ 0.1	0.0
Weighted average interest rates	1.10%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 113.6	55.7
Average amount outstanding (based on daily outstanding balances)	$ 79.1	$ 25.8
Weighted average interest rates	0.70%	0.40%