Debt (Credit Facilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Commercial paper:
Amount outstanding	$ 244.1	$ 159.8
Weighted average interest rates	0.90%	0.70%
Available credit facility capacity	$ 755.9	$ 840.2
IPL [Member]
Commercial paper:
Amount outstanding	0.0	0.0
Available credit facility capacity	300.0	300.0
WPL [Member]
Commercial paper:
Amount outstanding	$ 52.3	$ 19.9
Weighted average interest rates	0.70%	0.40%
Available credit facility capacity	$ 347.7	$ 380.1