Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Payables for purchased receivables	$ 33	$ 31
Deferred Compensation Liability, Classified, Noncurrent	84	86
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	19	17
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	18
Cash Surrender Value of Life Insurance	251	265
Corporate owned life insurance, borrowings	111	120
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Payables for purchased receivables	33	31
Cash Surrender Value of Life Insurance	120	129
Corporate owned life insurance, borrowings	$ 111	$ 120