Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			6 Months Ended
	Feb. 28, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2012	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 120			$ 120		$ 106
Provision for Callaway construction and operating license		69	$ 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		53			53		84
Ameren Illinois Company | Final Rate Order
Rate And Regulatory Matters [Line Items]
Loss Contingency, Estimate of Possible Loss		8			8
Current regulatory liabilities		6			6
Payments for Legal Settlements		2
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		58			58		$ 18
Provision for Callaway construction and operating license		69	$ 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%
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]
Earnings reduction in FERC allowed base return on common equity					$ 2