Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Basis Of Presentation And Significant Accounting Policies [Line Items]
Workforce Utilizing Remote Work Arrangement		50.00%	65.00%
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	26.00%				13.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 151				$ 65
Unrecorded Unconditional Purchase Obligation	1,993
Cash Surrender Value of Life Insurance	260			$ 264
Corporate owned life insurance, borrowings	$ 107			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	18.00%				9.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 49				$ 23
Unrecorded Unconditional Purchase Obligation	$ 1,300
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	35.00%				18.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 102				$ 42
Unrecorded Unconditional Purchase Obligation	648
Cash Surrender Value of Life Insurance	114			123
Corporate owned life insurance, borrowings	107			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	18
Equity Method Investments	34			$ 28
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 37