Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Federal) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 80	$ 106
Provision for Callaway construction and operating license		$ 69		$ 0
Midwest Independent Transmission System Operator, Inc [Member] | 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 [Member] | Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage		12.38%
Customer Requested Rate on Equity	8.67%	9.15%
Basis point reduction in FERC allowed base return on common equity		0.50%
Current regulatory liabilities		$ 45
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		39	84
Ameren Illinois Company | Midwest Independent Transmission System Operator, Inc [Member] | Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		32
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		28	18
Provision for Callaway construction and operating license		69	$ 0	$ 0
New Nuclear Energy Center COL [Member]
Rate And Regulatory Matters [Line Items]
Provision for Callaway construction and operating license		69
New Nuclear Energy Center COL [Member] | Union Electric Company
Rate And Regulatory Matters [Line Items]
Investments in Power and Distribution Projects		69
Provision for Callaway construction and operating license		69
Final Rate Order | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Payments for Legal Settlements		$ 8