Debt (Credit Facilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Commercial paper:
Amount outstanding	$ 337.4	$ 441.2
Weighted average interest rates	1.90%	2.80%
Available credit facility capacity	$ 662.6	$ 558.8
IPL [Member]
Commercial paper:
Amount outstanding	0.0	$ 50.4
Weighted average interest rates		2.80%
Available credit facility capacity	250.0	$ 199.6
WPL [Member]
Commercial paper:
Amount outstanding	$ 168.2	$ 105.5
Weighted average interest rates	1.80%	2.50%
Available credit facility capacity	$ 131.8	$ 244.5