Callaway Energy Center (Fair Values Of Investments In Debt And Equity Securities In Nuclear Decommissioning Trust Fund) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nuclear Waste Matters [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 262
Cash, cash equivalents, and restricted cash	176	$ 107	$ 68	$ 52
Fair Value	847	684
Cash and cash equivalents	16	16
Union Electric Company
Nuclear Waste Matters [Line Items]
Cost	450	420
Cash, cash equivalents, and restricted cash	39	8	$ 7	$ 5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	404	280
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	7	16
Fair Value	847	684
Cash and cash equivalents	9	0
Union Electric Company | Debt Securities
Nuclear Waste Matters [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	262	253
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	11	3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	4
Fair Value	273	252
Union Electric Company | Equity Securities
Nuclear Waste Matters [Line Items]
Available-for-sale Equity Securities, Amortized Cost Basis	183	162
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	393	277
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	7	12
Fair Value	569	427
Union Electric Company | Cash and Cash Equivalents
Nuclear Waste Matters [Line Items]
Cost	26	3
Cash, cash equivalents, and restricted cash	0	0
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Cash and cash equivalents	26	3
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	(21)	2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
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	$ (21)	$ 2