Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Commercial paper:
Amount outstanding	$ 307.9	$ 279.4
Weighted average remaining maturity (in days)	1 day
Weighted average interest rates	0.20%
Available credit facility capacity	692.1
Parent Company [Member]
Commercial paper:
Amount outstanding	149.9
Weighted average remaining maturity (in days)	1 day
Weighted average interest rates	0.20%
Available credit facility capacity	150.1
IPL [Member]
Commercial paper:
Amount outstanding	0
Available credit facility capacity	300.0
WPL [Member]
Commercial paper:
Amount outstanding	158.0	183.7
Weighted average remaining maturity (in days)	1 day
Weighted average interest rates	0.10%
Available credit facility capacity	$ 242.0