Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Basis Of Presentation And Significant Accounting Policies [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	2.1	0.3	1.4	0.4
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	$ 14		$ 14		$ 9
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	23		$ 23
State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			7.75%
Deferred Tax Liabilities, Net	97		$ 97
Other Affiliated Entities and Intercompany Eliminations | State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Increase (Decrease) in Income Taxes			14
Ameren Illinois Company | State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Increase (Decrease) in Income Taxes			1
Deferred Tax Liabilities, Net	$ 79		$ 79
Maximum | State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			9.50%
Minimum | State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			7.30%