Regulatory Matters (Narrative) (Details) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Regulatory Matters [Line Items]
Electric capacity of wind farm (in megawatts) | MW			1,000
Regulatory liabilities			$ 1,423.6	$ 1,493.2
IPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return			78.0
Regulatory liabilities			714.7	754.9
WPL [Member]
Regulatory Matters [Line Items]
Regulatory assets not earning a return			6.0
Regulatory liabilities			708.9	738.3
2020 Test Period Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase (decrease), amount			90.0
2020 Test Period Retail Electric [Member] | IPL [Member] | Tax-related [Member]
Regulatory Matters [Line Items]
Regulatory liabilities			27.0
2020 Test Period Retail Electric [Member] | IPL [Member] | Revenue Subject to Refund [Member]
Regulatory Matters [Line Items]
Regulatory liabilities			$ 8.0
2016 Test Year Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Interim rate increase (decrease), amount					$ 102.0
Authorized increase (decrease) in final rates, amount				$ 130.0
Forward Contracts [Member] | IUB [Member] | IPL [Member]
Regulatory Matters [Line Items]
Recovery period, authorized			10 years
DAEC PPA [Member]
Regulatory Matters [Line Items]
PPA term amendment			5 years
Recovery period, authorized			5 years
Forecast [Member] | 2020 Test Period Retail Electric [Member] | IPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		$ 127.0
Billing credit, period		12 months
Forecast [Member] | 2020 Test Period Retail Gas [Member] | IPL [Member]
Regulatory Matters [Line Items]
Authorized increase (decrease) in final rates, amount		$ 12.0
Forecast [Member] | DAEC PPA [Member]
Regulatory Matters [Line Items]
Buyout payment	$ 110.0