Debt (Credit Facilities) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Commercial paper:
Amount outstanding	$ 238.3	$ 159.8
Weighted average remaining maturity (in days)	4 days
Weighted average interest rates	0.60%
Available credit facility capacity	$ 761.7
Parent Company [Member]
Commercial paper:
Amount outstanding	$ 226.5
Weighted average remaining maturity (in days)	4 days
Weighted average interest rates	0.70%
Available credit facility capacity	$ 73.5
IPL [Member]
Commercial paper:
Amount outstanding	0.0
Available credit facility capacity	300.0
WPL [Member]
Commercial paper:
Amount outstanding	$ 11.8	$ 19.9
Weighted average remaining maturity (in days)	3 days
Weighted average interest rates	0.40%
Available credit facility capacity	$ 388.2