Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 242.6	$ 151.3
Average amount outstanding (based on daily outstanding balances)	$ 199.0	$ 127.9
Weighted average interest rates	0.60%	0.40%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 0.0	$ 1.4
Average amount outstanding (based on daily outstanding balances)	0.0	$ 0.0
Weighted average interest rates		0.40%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	55.7	$ 0.0
Average amount outstanding (based on daily outstanding balances)	$ 25.8	$ 0.0
Weighted average interest rates	0.40%