Rate And Regulatory Matters (Narrative-Illinois) (Detail) - USD ($) $ in Millions	1 Months Ended
	Apr. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Rate And Regulatory Matters [Line Items]
Regulatory assets		$ 1,330	$ 1,382
Current regulatory assets		146	260
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets		786	771
Current regulatory assets		98	167
2016 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets		12
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		$ 58	$ 103
Pending Rate Case | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 14