Common Equity (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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)		12,400,000.0
Proceeds from issuance of common stock, net		$ 149.6	$ 26.6	$ 151.2
Ownership percentage		15.00%
Other comprehensive income (loss)		$ (0.1)	0.0	0.2
IPL [Member]
Schedule of Common Equity [Line Items]
Retained earnings free of dividend restrictions		$ 679.0
Common equity ratio, computation of ratio average, number of months		13 months
Common equity ratio		42.00%
Restricted net assets of subsidiaries		$ 1,800.0
Other comprehensive income (loss)		0.0	0.0	0.0
WPL [Member]
Schedule of Common Equity [Line Items]
Retained earnings free of dividend restrictions		140.0
Restricted net assets of subsidiaries		1,700.0
Other comprehensive income (loss)		0.0	$ 0.0	0.0
At The Market Offering Program [Member]
Schedule of Common Equity [Line Items]
Maximum aggregate gross sales price of common stock that can be offered and sold		$ 125.0		$ 150.0
Common stock issued during the period, At-the-market offering programs (in shares)		3,074,931	0	4,373,234
Proceeds from issuance of common stock, net		$ 124.0		$ 133.0
Fees and commissions from issuance of common stock		$ 1.0		$ 2.0
Scenario, Forecast [Member] | WPL [Member]
Schedule of Common Equity [Line Items]
Retained earnings free of dividend restrictions	$ 140.0
Common equity ratio, computation of ratio average, number of months	13 months
Common equity ratio	51.00%