Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Narrative) (Details) - Subsequent Event [Member] - State and Local Jurisdiction [Member] $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate	7.75%
Increase (Decrease) in Income Taxes	$ 15
Deferred Tax Liabilities, Net	97
Other Affiliated Entities and Intercompany Eliminations [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Increase (Decrease) in Income Taxes	14
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Increase (Decrease) in Income Taxes	1
Deferred Tax Liabilities, Net	$ 79
Maximum
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate	9.50%
Minimum
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate	7.30%