Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 28, 2015	Mar. 31, 2016	Dec. 31, 2015
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 87	$ 80
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 55	$ 39
Midwest Independent Transmission System Operator, Inc | Administrative Law Judge [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage		10.32%
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage		12.38%
Customer Requested Rate on Equity	8.67%	9.15%
Current regulatory liabilities		$ 55
Incentive adder to FERC allowed base return on common equity		0.50%
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 37