Rate and Regulatory Matters (Narrative-Missouri) (Details) $ in Millions	2 Months Ended	3 Months Ended	12 Months Ended
	Feb. 22, 2021 USD ($) MWh	Mar. 31, 2021 MWh	Dec. 31, 2020 USD ($) MWh	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Public Utilities, General Disclosures [Line Items]
Purchased power			$ 513	$ 556	$ 581
Other operations and maintenance			1,661	1,745	1,772
Income tax benefit			155	182	237
Donations			25	12	33
Wind generation expenditures			564	0	0
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Purchased power			171	193	164
Other operations and maintenance			886	960	972
Income tax benefit			34	68	124
Donations			12	3	14
Wind generation expenditures			564	0	0
Final Rate Order | Union Electric Company | Electric
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			32
Reduction to annualized base level of net energy costs			115
Revenues			20
Purchased power			15
Other operations and maintenance			60
Income tax benefit			20
Revenue requirement change due to changes in MEEIA sales volumes			50
Amounts previously deferred under PISA			50
Donations			8
Final Rate Order | Union Electric Company | MEEIA 2019 [Domain] | Electric
Public Utilities, General Disclosures [Line Items]
Revenues			6
Incentive award if energy efficiency goals are achieved			11
Incentive award if energy efficiency goals are exceeded			1
MEEIA Energy efficiency investments for the 2022 program year			$ 70
Final Rate Order | Union Electric Company | MEEIA 2013 & 2016 | Electric
Public Utilities, General Disclosures [Line Items]
Revenues				$ 37	$ 11
Wind Generation Facility | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Amount of Megawatts | MWh			400
High Prairie Renewable Energy Center purchase price			$ 615
Minimum | Final Rate Order | Union Electric Company | Electric
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage			9.40%
Maximum | Final Rate Order | Union Electric Company | Electric
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage			9.80%
Subsequent Event | Wind Generation Facility | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Amount of Megawatts | MWh		300
Megawatts in service | MWh	120	150
Atchison Renewable Energy Center purchase price	$ 200