Rate And Regulatory Matters (Narrative-Illinois) (Details) $ in Millions	12 Months Ended
Dec. 31, 2015 USD ($) mi	Dec. 31, 2014 USD ($)
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 1,382	$ 1,582
Current regulatory assets	260	295
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	771	883
Current regulatory assets	167	129
Ameren Illinois Company \| 2015 IEMA Revenue Requirement Reconciliation [Member] \| IEIMA \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets	62
Ameren Illinois Company \| 2014 IEMA Revenue Requirement Reconciliation [Member] \| IEIMA \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Current regulatory assets	103
Ameren Illinois Company \| 2013 IEMA Revenue Requirement Reconciliation [Member] \| IEIMA \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Current regulatory assets		$ 65
Ameren Illinois Company \| Final Rate Order \| Gas Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 45
Public Utilities, Approved Return on Equity, Percentage	9.60%
Public Utilities, Approved Equity Capital Structure, Percentage	50.00%
Rate base	$ 1,200
Ameren Illinois Company \| Final Rate Order \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	15
Disallowed costs associated with debt redemption	15
Allowed Costs Associated with Debt Redemption	11
Ameren Illinois Company \| Final Rate Order \| IEIMA \| Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 106
Ameren Transmission Company of Illinois [Member] \| Spoon River Project [Member]
Rate And Regulatory Matters [Line Items]
Transmission Line Miles \| mi	46

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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