Common Equity (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018
Common Equity [Line Items]
Proceeds from issuance of common stock, net				$ 100.1	$ 137.3
IPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions		$ 693.0		693.0
Restricted net assets of subsidiaries		2,000.0		2,000.0
Comprehensive income		0.0	$ 0.0	0.0	0.0
WPL [Member]
Common Equity [Line Items]
Retained earnings free of dividend restrictions		70.0		70.0
Restricted net assets of subsidiaries		2,000.0		2,000.0
Comprehensive income		$ 0.0	$ 0.0	$ 0.0	$ 0.0
At The Market Offering Program [Member]
Common Equity [Line Items]
Common stock issued during the period, At-the-market offering program (in shares)				2,144,178
Proceeds from issuance of common stock, net				$ 88.0
Fees and commissions from issuance of common stock				$ 1.0
Scenario, Forecast [Member] | At The Market Offering Program [Member]
Common Equity [Line Items]
Maximum aggregate gross sales price of common stock that can be offered and sold						$ 175.0
Subsequent Event [Member] | At The Market Offering Program [Member]
Common Equity [Line Items]
Common stock issued during the period, At-the-market offering program (in shares)	550,000
Proceeds from issuance of common stock, net	$ 23.0