Derivative Financial Instruments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Derivative [Line Items]
Cash collateral held from counterparties	$ 2		$ 1
Counterparty letters of credit held as collateral	7		9
Derivative liabilities used as hedging instruments	404	[1],[2]	281	[1],[2]
Ameren Missouri [Member]
Derivative [Line Items]
Counterparty letters of credit held as collateral			1
Derivative liabilities used as hedging instruments	45	[2]	37	[2]
Genco [Member]
Derivative [Line Items]
Counterparty letters of credit held as collateral			1
Derivative liabilities used as hedging instruments	14	[2]	3	[2]
Ameren Illinois Company [Member]
Derivative [Line Items]
Derivative liabilities used as hedging instruments	365	[2]	386	[2]
Mark To Market Derivative Liabilities [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Derivative liabilities used as hedging instruments	$ 114		$ 200

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

[2] The derivative asset and liability balances are presented net of counterparty credit considerations.