Debt (Credit Facilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Commercial paper:
Amount outstanding	$ 159.8	$ 141.3
Weighted average interest rates	0.70%	0.40%
Weighted average remaining maturity	4 days	4 days
Available credit facility capacity	$ 840.2	$ 858.7
IPL [Member]
Commercial paper:
Amount outstanding	0.0	0.0
Available credit facility capacity	300.0	300.0
WPL [Member]
Commercial paper:
Amount outstanding	$ 19.9	0.0
Weighted average interest rates	0.40%
Weighted average remaining maturity	4 days
Available credit facility capacity	$ 380.1	$ 400.0