Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Commercial paper:
Amount outstanding	$ 353.8	$ 279.4
Weighted average remaining maturity (in days)	3 days
Weighted average interest rates	0.20%
Available credit facility capacity	646.2
Parent Company [Member]
Commercial paper:
Amount outstanding	169.1
Weighted average remaining maturity (in days)	3 days
Weighted average interest rates	0.30%
Available credit facility capacity	130.9
IPL [Member]
Commercial paper:
Amount outstanding	38.0	0
Weighted average remaining maturity (in days)	1 day
Weighted average interest rates	0.30%
Available credit facility capacity	262.0
WPL [Member]
Commercial paper:
Amount outstanding	146.7	183.7
Weighted average remaining maturity (in days)	3 days
Weighted average interest rates	0.10%
Available credit facility capacity	$ 253.3