Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	$ 20		$ 20		$ 17
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	17		17
Payables for purchased receivables	40		40		31
Cash Surrender Value of Life Insurance	256		256		265
Corporate owned life insurance, borrowings	113		113		120
Deferred Compensation Liability, Classified, Noncurrent	$ 86		$ 86		86
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements (in shares)	2.2	2.1	1.9	1.4
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Payables for purchased receivables	$ 40		$ 40		31
Cash Surrender Value of Life Insurance	120		120		129
Corporate owned life insurance, borrowings	$ 113		$ 113		$ 120