Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	27.00%		21.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 137		$ 99
Noncontrolling Interest in Variable Interest Entity	41	$ 37
Cash Surrender Value of Life Insurance	275	272
Corporate owned life insurance, borrowings	$ 107	107
Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	19.00%		16.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 33		$ 29
Union Electric Company | Final Rate Order
Basis Of Presentation And Significant Accounting Policies [Line Items]
COVID-19 pandemic costs	$ 9
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	32.00%		25.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 104		$ 70
Cash Surrender Value of Life Insurance	117	115
Corporate owned life insurance, borrowings	107	$ 107
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 33