Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 446.5	$ 397.6	$ 446.5	$ 397.6
Average amount outstanding (based on daily outstanding balances)	$ 234.5	$ 307.8	$ 272.1	$ 292.3
Weighted average interest rates	2.20%	1.10%	2.00%	1.00%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 31.4	$ 14.6	$ 31.4	$ 14.6
Average amount outstanding (based on daily outstanding balances)	$ 5.1	$ 1.0	$ 2.6	$ 0.6
Weighted average interest rates	2.30%	1.20%	2.30%	1.20%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 109.4	$ 212.5	$ 109.4	$ 212.5
Average amount outstanding (based on daily outstanding balances)	$ 45.2	$ 134.9	$ 28.4	$ 107.2
Weighted average interest rates	1.80%	1.00%	1.80%	0.90%