Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Common Equity [Line Items]
Shares available for issuance under the Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan (in shares)		12,100,000
Proceeds from issuance of common stock, net		$ 390.3	$ 196.6	$ 149.6
Other comprehensive income (loss)		(0.4)	2.2	(0.1)
IPL [Member]
Schedule of Common Equity [Line Items]
Other comprehensive income (loss)		0.0	0.0	0.0
WPL [Member]
Schedule of Common Equity [Line Items]
Other comprehensive income (loss)		$ 0.0	$ 0.0	$ 0.0
Equity Forward Agreements [Member]
Schedule of Common Equity [Line Items]
Common stock issued during the period (in shares)		8,358,973	0	0
Proceeds from issuance of common stock, net		$ 366.0
Initial forward sale price (in dollars per share)		$ 43.75
Shares included in diluted earnings per share (in shares)		67,689
At-the-market Offering Program [Member]
Schedule of Common Equity [Line Items]
Common stock issued during the period (in shares)		0	4,171,013	3,074,931
Proceeds from issuance of common stock, net			$ 173.0	$ 124.0
Fees and commissions from issuance of common stock			2.0	1.0
Maximum aggregate gross sales price of common stock that can be offered and sold			$ 175.0	$ 125.0
Forecast [Member] | Equity Forward Agreements [Member]
Schedule of Common Equity [Line Items]
Common stock issued during the period (in shares)	4,275,127
Initial forward sale price (in dollars per share)	$ 52.235