Consolidated Balance Sheet (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Current Assets:
Cash and cash equivalents	$ 117	$ 255
Accounts receivable - trade (less allowance for doubtful accounts )	417	473
Unbilled revenue	374	324
Miscellaneous accounts and notes receivable	69	69
Materials and supplies	686	712
Mark-to-market derivative assets	156	115
Current regulatory assets	236	215
Other current assets	99	132
Total current assets	2,154	2,295
Property and Plant, Net	17,690	18,127
Investments and Other Assets:
Nuclear decommissioning trust fund	386	357
Goodwill	411	411
Intangible assets	12	7
Regulatory assets	1,551	1,603
Other assets	776	845
Total investments and other assets	3,136	3,223
TOTAL ASSETS	22,980	23,645
Current Liabilities:
Current maturities of long-term debt	179	179
Short-term debt	30	148
Accounts and wages payable	479	693
Taxes accrued	141	65
Interest accrued	113	101
Customer deposits	97	98
Mark-to-market derivative liabilities	198	161
Current regulatory liabilities	145	133
Other current liabilities	221	207
Total current liabilities	1,603	1,785
Long-term Debt, Net	6,678	6,677
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	3,199	3,315
Accumulated deferred investment tax credits	75	79
Regulatory liabilities	1,470	1,502
Asset retirement obligations	440	428
Pension and other postretirement benefits	1,251	1,344
Other deferred credits and liabilities	564	447
Total deferred credits and other liabilities	6,999	7,115
Commitments and Contingencies
Ameren Corporation Stockholders' Equity:
Common stock	2	2
Other paid-in capital	5,600	5,598
Retained earnings	1,983	2,369
Accumulated other comprehensive income (loss)	(30)	(50)
Total stockholders' equity	7,555	7,919
Noncontrolling Interest	145	149
Total equity	7,700	8,068
TOTAL LIABILITIES AND EQUITY	22,980	23,645
Union Electric Company [Member]
Current Assets:
Cash and cash equivalents		201
Accounts receivable - trade (less allowance for doubtful accounts )	180	212
Unbilled revenue	224	139
Miscellaneous accounts and notes receivable	46	43
Materials and supplies	388	348
Current regulatory assets	119	109
Other current assets	66	82
Total current assets	1,023	1,134
Property and Plant, Net	10,038	9,958
Investments and Other Assets:
Nuclear decommissioning trust fund	386	357
Intangible assets	11	7
Regulatory assets	775	855
Other assets	447	446
Total investments and other assets	1,619	1,665
TOTAL ASSETS	12,680	12,757
Current Liabilities:
Current maturities of long-term debt	178	178
Borrowings from money pool	67
Accounts and wages payable	168	414
Accounts payable - affiliates	62	73
Taxes accrued	103	74
Interest accrued	76	62
Current regulatory liabilities	46	57
Other current liabilities	130	84
Total current liabilities	830	942
Long-term Debt, Net	3,772	3,772
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,199	2,132
Accumulated deferred investment tax credits	67	70
Regulatory liabilities	879	836
Asset retirement obligations	337	328
Pension and other postretirement benefits	442	491
Other deferred credits and liabilities	153	149
Total deferred credits and other liabilities	4,077	4,006
Commitments and Contingencies
Ameren Corporation Stockholders' Equity:
Common stock	511	511
Other paid-in capital	1,555	1,555
Preferred stock not subject to mandatory redemption	80	80
Retained earnings	1,855	1,891
Total stockholders' equity	4,001	4,037
TOTAL LIABILITIES AND EQUITY	12,680	12,757
Ameren Illinois Company [Member]
Current Assets:
Cash and cash equivalents	60	21
Advances to money pool	67
Accounts receivable - trade (less allowance for doubtful accounts )	175	201
Accounts receivable - affiliates	10	15
Unbilled revenue	118	146
Miscellaneous accounts and notes receivable	9	6
Materials and supplies	137	199
Counterparty collateral asset	45	50
Current regulatory assets	231	306
Current accumulated deferred income taxes, net	44	58
Other current assets	10	15
Total current assets	906	1,017
Property and Plant, Net	4,869	4,770
Investments and Other Assets:
Tax receivable - Genco	49	56
Goodwill	411	411
Regulatory assets	775	748
Other assets	117	211
Total investments and other assets	1,352	1,426
TOTAL ASSETS	7,127	7,213
Current Liabilities:
Current maturities of long-term debt	1	1
Accounts and wages payable	171	133
Accounts payable - affiliates	76	103
Taxes accrued	14	15
Customer deposits	75	76
Mark-to-market derivative liabilities	97	99
Mark-to-market derivative liabilities - affiliates	114	200
Environmental remediation	32	63
Current regulatory liabilities	98	76
Other current liabilities	86	92
Total current liabilities	764	858
Long-term Debt, Net	1,657	1,657
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	941	895
Accumulated deferred investment tax credits	6	7
Regulatory liabilities	591	666
Pension and other postretirement benefits	458	495
Other deferred credits and liabilities	276	183
Total deferred credits and other liabilities	2,272	2,246
Commitments and Contingencies
Ameren Corporation Stockholders' Equity:
Common stock
Other paid-in capital	1,965	1,965
Preferred stock not subject to mandatory redemption	62	62
Retained earnings	392	408
Accumulated other comprehensive income (loss)	15	17
Total stockholders' equity	2,434	2,452
TOTAL LIABILITIES AND EQUITY	7,127	7,213
Ameren Energy Generating Company [Member]
Current Assets:
Cash and cash equivalents	26	8
Advances to money pool	38	74
Accounts receivable - affiliates	73	89
Miscellaneous accounts and notes receivable	10	13
Materials and supplies	121	122
Other current assets	19	19
Total current assets	287	325
Property and Plant, Net	2,274	2,231
Investments and Other Assets:
Other assets	14	16
TOTAL ASSETS	2,575	2,572
Current Liabilities:
Accounts and wages payable	82	71
Accounts payable - affiliates	11	13
Current portion of tax payable - Ameren Illinois	9	8
Taxes accrued	20	20
Interest accrued	13	13
Other current liabilities	22	17
Total current liabilities	157	142
Long-term Debt, Net	824	824
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	296	304
Accumulated deferred investment tax credits	2	2
Tax payable - Ameren Illinois	49	56
Asset retirement obligations	68	66
Pension and other postretirement benefits	136	141
Other deferred credits and liabilities	21	12
Total deferred credits and other liabilities	572	581
Commitments and Contingencies
Ameren Corporation Stockholders' Equity:
Common stock
Other paid-in capital	654	653
Retained earnings	432	437
Accumulated other comprehensive income (loss)	(67)	(72)
Total stockholders' equity	1,019	1,018
Noncontrolling Interest	3	7
Total equity	1,022	1,025
TOTAL LIABILITIES AND EQUITY	$ 2,575	$ 2,572