Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Payables for purchased receivables	$ 40		$ 40		$ 31
Deferred Compensation Liability, Classified, Noncurrent	85		85		86
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	17		17		17
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	19		19
Cash Surrender Value of Life Insurance	249		249		265
Corporate owned life insurance, borrowings	$ 107		$ 107		120
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	2.0	0.8	1.8	1.1
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	117		117		129
Corporate owned life insurance, borrowings	111		$ 111		$ 120
State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			4.00%
Deferred Tax Liabilities, Net	33		$ 33
State and Local Jurisdiction [Member] | Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			4.00%
Deferred Tax Liabilities, Net	$ 33		$ 33
State and Local Jurisdiction [Member] | Maximum
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			6.25%
State and Local Jurisdiction [Member] | Maximum | Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			6.25%