Callaway Energy Center (Fair Values Of Investments In Debt And Equity Securities In Nuclear Decommissioning Trust Fund) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nuclear Waste Matters [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 272
Cash, cash equivalents, and restricted cash	301	$ 176	$ 107	$ 68
Fair Value	982	847
Cash and cash equivalents	139	16
Union Electric Company
Nuclear Waste Matters [Line Items]
Cost	475	450
Cash, cash equivalents, and restricted cash	145	39	$ 8	$ 7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	516	404
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	9	7
Fair Value	982	847
Cash and cash equivalents	136	9
Cost	982	847
Union Electric Company | Debt securities
Nuclear Waste Matters [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	272	262
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	25	11
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	297	273
Union Electric Company | Equity securities
Nuclear Waste Matters [Line Items]
Available-for-sale Equity Securities, Amortized Cost Basis	198	183
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	491	393
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	9	7
Equity Securities, FV-NI	680	569
Union Electric Company | Cash and cash equivalents
Nuclear Waste Matters [Line Items]
Marketable Securities	4	26
Cash, cash equivalents, and restricted cash	0	0
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Cash and cash equivalents	4	26
Union Electric Company | Other Debt And Equity Securities
Nuclear Waste Matters [Line Items]
Net Receivables (Payables) From Pending Securities Sales, Interest, and Securities Purchases, Cost Basis	1	(21)
Net Receivables (Payables) From Pending Securities Sales, Interest, And Securities Purchases, Available-For-sale Securities, Accumulated Gross Unrealized Loss, Before Tax	0	0
Net Receivables (Payables) From Pending Securities Sales, Interest, and Securities Purchases, Fair Value	1	(21)
Net Receivables (Payables) From Pending Securities Sales, Interest, And Securities Purchases, Available-For-sale Securities, Accumulated Gross Unrealized Gain, Before Tax	$ 0	$ 0