Debt (Credit Facilities) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Commercial paper:
Amount outstanding	$ 109.1	$ 141.3
Weighted average remaining maturity (in days)	3 days
Weighted average interest rates	0.40%
Available credit facility capacity	$ 890.9
Parent Company [Member]
Commercial paper:
Amount outstanding	$ 109.1
Weighted average remaining maturity (in days)	3 days
Weighted average interest rates	0.40%
Available credit facility capacity	$ 190.9
IPL [Member]
Commercial paper:
Amount outstanding	0.0
Available credit facility capacity	300.0
WPL [Member]
Commercial paper:
Amount outstanding	0.0
Available credit facility capacity	$ 400.0