Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Common Equity [Line Items]
Proceeds from issuance of common stock, net	$ 124.8	$ 91.2	$ 185.4	$ 191.3
Comprehensive income (loss)		(0.3)	(0.7)	0.3
IPL [Member]
Common Equity [Line Items]
Comprehensive income (loss)	0.0	0.0	0.0	0.0
WPL [Member]
Common Equity [Line Items]
Comprehensive income (loss)	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Equity Forward Agreements [Member]
Common Equity [Line Items]
Common stock issued during the period, equity forward agreements (in shares)			3,790,300
Forward sales price (in dollars per share)	$ 43.99		$ 43.99			$ 44.33
Proceeds from issuance of common stock, net			$ 167.0
Shares included in diluted earnings per share (in shares)			671,237
Forecast [Member] | Equity Forward Agreements [Member]
Common Equity [Line Items]
Common stock issued during the period, equity forward agreements (in shares)					8,358,973