Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 395	[1],[2]	$ 358	[1],[3]
Assets fair value	622	[1]	596	[1]
Derivative liabilities	404	[1],[4]	281	[1],[4]
Excluded receivables, payables, and accrued income, net	(9)		(1)
Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	395	[2]	358	[3]
Assets fair value	450	[2]	413	[3]
Derivative liabilities	45	[4]	37	[4]
Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			79	[4]
Derivative liabilities	365	[4]	386	[4]
Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	[4]	13	[4]
Derivative liabilities	14	[4]	3	[4]
Commodity Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	227	[1],[4]	238	[1],[4]
Commodity Contracts [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	55	[4]	55	[4]
Commodity Contracts [Member] | Fuel Oils [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	18	[1],[4]	37	[1],[4]
Derivative liabilities	11	[1],[4]	2	[1],[4]
Commodity Contracts [Member] | Fuel Oils [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	[4]	23	[4]
Derivative liabilities	4	[4]	1	[4]
Commodity Contracts [Member] | Fuel Oils [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	[4]	11	[4]
Derivative liabilities	5	[4]	1	[4]
Commodity Contracts [Member] | Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	[1],[4]	6	[1],[4]
Derivative liabilities	168	[1],[4]	198	[1],[4]
Commodity Contracts [Member] | Natural Gas [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[4]	2	[4]
Derivative liabilities	23	[4]	26	[4]
Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[4]	2	[4]
Derivative liabilities	144	[4]	169	[4]
Commodity Contracts [Member] | Natural Gas [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	[4]	2	[4]
Derivative liabilities	1	[4]	2	[4]
Commodity Contracts [Member] | Power [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	199	[1],[4]	195	[1],[4]
Derivative liabilities	214	[1],[4]	80	[1],[4]
Commodity Contracts [Member] | Power [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	41	[4]	30	[4]
Derivative liabilities	17	[4]	9	[4]
Commodity Contracts [Member] | Power [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			77	[4]
Derivative liabilities	221	[4]	217	[4]
Commodity Contracts [Member] | Coal [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	10	[1],[4]
Commodity Contracts [Member] | Coal [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	8	[4]
Commodity Contracts [Member] | Uranium [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[1],[4]	1	[1],[4]
Commodity Contracts [Member] | Uranium [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[4]	1	[4]
Cash and Cash Equivalents [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	4	[1],[2]	3	[1],[3]
Cash and Cash Equivalents [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	4	[2]	3	[3]
Equity Securities [Member] | U.S. Large Capitalization [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	247	[1],[2]	234	[1],[3]
Equity Securities [Member] | U.S. Large Capitalization [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	247	[2]	234	[3]
Debt Securities [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	43	[1],[2]	44	[1],[3]
Debt Securities [Member] | Corporate Debt Securities [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	43	[2]	44	[3]
Debt Securities [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1	[1],[2]	1	[1],[3]
Debt Securities [Member] | Municipal Bonds [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1	[2]	1	[3]
Debt Securities [Member] | US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	87	[1],[2]	65	[1],[3]
Debt Securities [Member] | US Treasury and Government [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	87	[2]	65	[3]
Debt Securities [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	12	[1],[2]	10	[1],[3]
Debt Securities [Member] | Asset-backed Securities [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	12	[2]	10	[3]
Debt Securities [Member] | Other Debt Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1	[1],[2]	1	[1],[3]
Debt Securities [Member] | Other Debt Obligations [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1	[2]	1	[3]
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	251	[1],[2]	237	[1],[3]
Assets fair value	268	[1]	274	[1]
Derivative liabilities	34	[1],[4]	24	[1],[4]
Fair Value, Inputs, Level 1 [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	251	[2]	237	[3]
Assets fair value	260	[2]	259	[3]
Derivative liabilities	13	[4]	13	[4]
Fair Value, Inputs, Level 1 [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	2	[4]	7	[4]
Fair Value, Inputs, Level 1 [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	[4]	12	[4]
Derivative liabilities	14	[4]	3	[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	17	[1],[4]	37	[1],[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	9	[4]	22	[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Fuel Oils [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	[1],[4]	33	[1],[4]
Derivative liabilities	9	[1],[4]	2	[1],[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	[4]	20	[4]
Derivative liabilities	2	[4]	1	[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	[4]	10	[4]
Derivative liabilities	5	[4]	1	[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	[1],[4]	4	[1],[4]
Derivative liabilities	15	[1],[4]	22	[1],[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[4]	2	[4]
Derivative liabilities	11	[4]	12	[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	2	[4]	7	[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[4]	2	[4]
Derivative liabilities	1	[4]	2	[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Coal [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	10	[1],[4]
Fair Value, Inputs, Level 1 [Member] | Commodity Contracts [Member] | Coal [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	8	[4]
Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	4	[1],[2]	3	[1],[3]
Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	4	[2]	3	[3]
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | U.S. Large Capitalization [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	247	[1],[2]	234	[1],[3]
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | U.S. Large Capitalization [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	247	[2]	234	[3]
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	144	[1],[2]	121	[1],[3]
Assets fair value	166	[1]	123	[1]
Derivative liabilities	175	[1],[4]	2	[1],[4]
Fair Value, Inputs, Level 2 [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	144	[2]	121	[3]
Assets fair value	154	[2]	122	[3]
Derivative liabilities	24	[4]	1	[4]
Fair Value, Inputs, Level 2 [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	142	[4]
Fair Value, Inputs, Level 2 [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	[4]
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	22	[1],[4]	2	[1],[4]
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	[4]	1	[4]
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member] | Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	[1],[4]
Derivative liabilities	153	[1],[4]
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	12	[4]
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[4]
Derivative liabilities	142	[4]
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	[4]
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member] | Power [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	17	[1],[4]	2	[1],[4]
Derivative liabilities	22	[1],[4]	2	[1],[4]
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member] | Power [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	[4]	1	[4]
Derivative liabilities	12	[4]	1	[4]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	43	[1],[2]	44	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Corporate Debt Securities [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	43	[2]	44	[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1	[1],[2]	1	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Municipal Bonds [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1	[2]	1	[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	87	[1],[2]	65	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | US Treasury and Government [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	87	[2]	65	[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	12	[1],[2]	10	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Asset-backed Securities [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	12	[2]	10	[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Other Debt Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1	[1],[2]	1	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Other Debt Obligations [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1	[2]	1	[3]
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets fair value	188	[1]	199	[1]
Derivative liabilities	195	[1],[4]	255	[1],[4]
Fair Value, Inputs, Level 3 [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets fair value	36	[2]	32	[3]
Derivative liabilities	8	[4]	23	[4]
Fair Value, Inputs, Level 3 [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			79	[4]
Derivative liabilities	221	[4]	379	[4]
Fair Value, Inputs, Level 3 [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[4]	1	[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	188	[1],[4]	199	[1],[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	36	[4]	32	[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Fuel Oils [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	[1],[4]	4	[1],[4]
Derivative liabilities	2	[1],[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	[4]	3	[4]
Derivative liabilities	2	[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[4]	1	[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Natural Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			2	[1],[4]
Derivative liabilities			176	[1],[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities			14	[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			2	[4]
Derivative liabilities			162	[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Power [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	182	[1],[4]	193	[1],[4]
Derivative liabilities	192	[1],[4]	78	[1],[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Power [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	31	[4]	29	[4]
Derivative liabilities	5	[4]	8	[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Power [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			77	[4]
Derivative liabilities	221	[4]	217	[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Uranium [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[1],[4]	1	[1],[4]
Fair Value, Inputs, Level 3 [Member] | Commodity Contracts [Member] | Uranium [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	$ 1	[4]	$ 1	[4]

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

[2] Balance excludes $(9) million of receivables, payables, and accrued income, net.

[3] Balance excludes $(1) million of receivables, payables, and accrued income, net.

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