Rate And Regulatory Matters (Narrative-Illinois) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Regulatory assets	$ 1,437	$ 1,382
Current regulatory assets	149	260
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	816	771
Current regulatory assets	$ 108	167
FEJA [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Equity Capital Structure, Percentage	50.00%
Public Utilities, Approved Return on Equity, Percentage	5.80%
Increase Decrease in Return on Equity for Energy Savings Goals	2.00%
FEJA 2018 through 2021 [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
FEJA Energy Efficiency Investments	$ 100
FEJA 2022 through 2025 [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
FEJA Energy Efficiency Investments	107
FEJA 2026 through 2030 [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
FEJA Energy Efficiency Investments	114
IEIMA \| 2016 IEIMA Revenue Requirement Reconciliation [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	23
IEIMA \| 2015 IEMA Revenue Requirement Reconciliation [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	68
IEIMA \| 2014 IEMA Revenue Requirement Reconciliation [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		$ 103
IEIMA \| Final Rate Order \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 14

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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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