Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 336.4	$ 325.5
Average amount outstanding (based on daily outstanding balances)	$ 310.1	$ 276.5
Weighted average interest rates	1.90%	0.90%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 3.0	$ 9.2
Average amount outstanding (based on daily outstanding balances)	$ 0.0	$ 0.1
Weighted average interest rates	1.80%	1.10%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 36.7	$ 113.6
Average amount outstanding (based on daily outstanding balances)	$ 11.5	$ 79.1
Weighted average interest rates	1.60%	0.70%