Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 152.6	$ 311.6	$ 152.6	$ 316.2
Average amount outstanding (based on daily outstanding balances)	$ 93.2	$ 262.7	$ 110.5	$ 269.1
Weighted average interest rates	0.50%	0.20%	0.40%	0.20%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 9.7	$ 6.0	$ 9.7	$ 10.0
Average amount outstanding (based on daily outstanding balances)	$ 0.1	$ 0.1	$ 0.1	$ 0.2
Weighted average interest rates	0.50%	0.20%	0.50%	0.20%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 0.0	$ 168.4	$ 0.0	$ 204.7
Average amount outstanding (based on daily outstanding balances)	$ 0.0	$ 141.8	$ 0.0	$ 157.3
Weighted average interest rates		0.10%		0.10%