Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019
Basis Of Presentation And Significant Accounting Policies [Line Items]
Workforce Utilizing Remote Work Arrangement		65.00%
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	26.00%			17.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 132			$ 83
Unrecorded Unconditional Purchase Obligation	2,062
Cash Surrender Value of Life Insurance	257		$ 264
Corporate owned life insurance, borrowings	$ 110		114
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%			11.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 42			$ 24
Unrecorded Unconditional Purchase Obligation	$ 1,348
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	33.00%			25.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 90			$ 59
Unrecorded Unconditional Purchase Obligation	667
Cash Surrender Value of Life Insurance	117		123
Corporate owned life insurance, borrowings	110		114
Variable Interest Entity, Not Primary Beneficiary
Basis Of Presentation And Significant Accounting Policies [Line Items]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	17
Equity Method Investments	33		$ 28
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 38