Debt (Credit Facilities) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Commercial paper:
Amount outstanding	$ 213.4	$ 159.8
Weighted average remaining maturity (in days)	2 days
Weighted average interest rates	0.60%
Available credit facility capacity	$ 786.6
Parent Company [Member]
Commercial paper:
Amount outstanding	$ 187.9
Weighted average remaining maturity (in days)	2 days
Weighted average interest rates	0.60%
Available credit facility capacity	$ 112.1
IPL [Member]
Commercial paper:
Amount outstanding	0.0
Available credit facility capacity	300.0
WPL [Member]
Commercial paper:
Amount outstanding	$ 25.5	$ 19.9
Weighted average remaining maturity (in days)	1 day
Weighted average interest rates	0.40%
Available credit facility capacity	$ 374.5