Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Commercial paper:
Amount outstanding	$ 221.3	$ 279.4
Weighted average remaining maturity (in days)	1 day
Weighted average interest rates	0.10%
Available credit facility capacity	778.7
Parent Company [Member]
Commercial paper:
Amount outstanding	65.8
Weighted average remaining maturity (in days)	1 day
Weighted average interest rates	0.20%
Available credit facility capacity	234.2
IPL [Member]
Commercial paper:
Amount outstanding	0
Available credit facility capacity	300.0
WPL [Member]
Commercial paper:
Amount outstanding	155.5	183.7
Weighted average remaining maturity (in days)	2 days
Weighted average interest rates	0.10%
Available credit facility capacity	$ 244.5