Rate And Regulatory Matters (Narrative-Illinois) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2016	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Rate And Regulatory Matters [Line Items]
Regulatory assets			$ 1,312	$ 1,382
Current regulatory assets			107	260
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets			791	771
Current regulatory assets			59	167
2016 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets			23
2015 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets			66
2014 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Current regulatory assets			$ 22	$ 103
Pending Rate Case | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 14
Subsequent Event | Administrative Law Judge | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 14