Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 87		$ 87		$ 80
Provision for Callaway construction and operating license		0	$ 0	0	$ 69
New Nuclear Energy Center COL [Member]
Rate And Regulatory Matters [Line Items]
Provision for Callaway construction and operating license					69
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		56		56		39
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		11		11		$ 28
Provision for Callaway construction and operating license		0	0	$ 0	69
Union Electric Company | New Nuclear Energy Center COL [Member]
Rate And Regulatory Matters [Line Items]
Investments in Power and Distribution Projects			$ 69		69
Provision for Callaway construction and operating license					$ 69
Midwest Independent Transmission System Operator, Inc | Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage				10.32%
Midwest Independent Transmission System Operator, Inc | Administrative Law Judge
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage				9.70%
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%
Incentive adder to FERC allowed base return on common equity				0.50%
Current regulatory liabilities		61		$ 61
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 42		$ 42
Maximum | Midwest Independent Transmission System Operator, Inc | Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage				10.82%