Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Common Equity [Line Items]
Proceeds from issuance of common stock, net		$ 6.0	$ 93.8	$ 60.6	$ 100.1
Comprehensive income (loss)		(1.4)	0.6	(0.7)	0.6
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)				1,090,300
Forward sales price (in dollars per share)		$ 44.13		$ 44.13			$ 44.33
Proceeds from issuance of common stock, net				$ 48.0
Shares included in diluted earnings per share (in shares)				559,597
Forecast [Member] | Equity Forward Agreements [Member]
Common Equity [Line Items]
Common stock issued during the period, equity forward agreements (in shares)						8,358,973
Subsequent Event [Member] | Equity Forward Agreements [Member]
Common Equity [Line Items]
Common stock issued during the period, equity forward agreements (in shares)	1,500,000
Forward sales price (in dollars per share)	$ 44.06
Proceeds from issuance of common stock, net	$ 66.0