Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 89		$ 89		$ 106
Provision for Callaway construction and operating license		0	$ 0	69	$ 0
New Nuclear Energy Center COL
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		37		37		84
Ameren Illinois Company | Final Rate Order
Rate And Regulatory Matters [Line Items]
Payments for Legal Settlements				8
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		41		41		$ 18
Provision for Callaway construction and operating license		0	$ 0	69	$ 0
Union Electric Company | New Nuclear Energy Center COL
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 | Pending Ferc Case
Rate And Regulatory Matters [Line Items]
Customer Requested Rate on Equity	8.67%			9.15%
Public Utilities, Approved Return on Equity, Percentage				12.38%
Current regulatory liabilities		36		$ 36
Basis point reduction in FERC allowed base return on common equity				0.50%
Earnings reduction in FERC allowed base return on common equity				$ 4
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		$ 25		$ 25
Earnings reduction in FERC allowed base return on common equity				$ 2