Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Common Equity [Line Items]
Proceeds from issuance of common stock, net	$ 54.6	$ 6.3
Comprehensive income	0.7
IPL [Member]
Common Equity [Line Items]
Comprehensive income	0.0	0.0
WPL [Member]
Common Equity [Line Items]
Comprehensive income	$ 0.0	$ 0.0
Equity Forward Agreements [Member]
Common Equity [Line Items]
Common stock issued during the period, equity forward agreements (in shares)	1,090,300
Forward sales price (in dollars per share)	$ 44.13			$ 44.33
Proceeds from issuance of common stock, net	$ 48.0
Shares included in diluted earnings per share (in shares)	96,261
Scenario, Forecast [Member] | Equity Forward Agreements [Member]
Common Equity [Line Items]
Common stock issued during the period, equity forward agreements (in shares)			8,358,973