Rate And Regulatory Matters (Narrative-Illinois) (Detail) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Rate And Regulatory Matters [Line Items]
Regulatory Assets, Noncurrent		$ 1,134	$ 1,127
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory Assets, Noncurrent		776	$ 759
Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Estimated Capital Project Costs		81
Property, Plant and Equipment, Additions		38
2019 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory Assets, Noncurrent		$ 41
Subsequent Event | Pending Rate Case [Member] | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 7