Consolidated Balance Sheet (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Current Assets:
Cash and cash equivalents	$ 208	$ 255
Accounts receivable - trade (less allowance for doubtful accounts)	446	473
Unbilled revenue	232	324
Miscellaneous accounts and notes receivable	65	69
Materials and supplies	625	712
Mark-to-market derivative assets	167	115
Current regulatory assets	247	215
Other current assets	134	132
Total current assets	2,124	2,295
Property and Plant, Net	17,535	18,127
Investments and Other Assets:
Nuclear decommissioning trust fund	390	357
Goodwill	411	411
Intangible assets	9	7
Regulatory assets	1,657	1,603
Other assets	773	845
Total investments and other assets	3,240	3,223
TOTAL ASSETS	22,899	23,645
Current Liabilities:
Current maturities of long-term debt	179	179
Short-term debt	126	148
Accounts and wages payable	366	693
Taxes accrued	101	65
Interest accrued	149	101
Customer deposits	98	98
Mark-to-market derivative liabilities	220	161
Current regulatory liabilities	138	133
Other current liabilities	237	207
Total current liabilities	1,614	1,785
Long-term Debt, Net	6,677	6,677
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	3,111	3,315
Accumulated deferred investment tax credits	77	79
Regulatory liabilities	1,483	1,502
Asset retirement obligations	434	428
Pension and other postretirement benefits	1,357	1,344
Other deferred credits and liabilities	567	447
Total deferred credits and other liabilities	7,029	7,115
Commitments and Contingencies (Notes 2, 8, 9 and 10)
Stockholders' Equity:
Common stock	2	2
Other paid-in capital, principally premium on common stock	5,596	5,598
Retained earnings	1,869	2,369
Accumulated other comprehensive income (loss)	(35)	(50)
Total stockholders' equity	7,432	7,919
Noncontrolling Interests	147	149
Total equity	7,579	8,068
TOTAL LIABILITIES AND EQUITY	22,899	23,645
Union Electric Company [Member]
Current Assets:
Cash and cash equivalents	3	201
Accounts receivable - trade (less allowance for doubtful accounts)	170	212
Accounts receivable - affiliates	3	1
Unbilled revenue	110	139
Miscellaneous accounts and notes receivable	41	42
Materials and supplies	365	348
Mark-to-market derivative assets	59	49
Current regulatory assets	113	109
Other current assets	24	33
Total current assets	888	1,134
Property and Plant, Net	9,976	9,958
Investments and Other Assets:
Nuclear decommissioning trust fund	390	357
Intangible assets	9	7
Regulatory assets	842	855
Other assets	441	446
Total investments and other assets	1,682	1,665
TOTAL ASSETS	12,546	12,757
Current Liabilities:
Current maturities of long-term debt	178	178
Accounts and wages payable	142	414
Accounts payable - affiliates	107	73
Taxes accrued	113	74
Interest accrued	58	62
Current regulatory liabilities	60	57
Other current liabilities	120	84
Total current liabilities	778	942
Long-term Debt, Net	3,772	3,772
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,115	2,132
Accumulated deferred investment tax credits	68	70
Regulatory liabilities	874	836
Asset retirement obligations	333	328
Pension and other postretirement benefits	498	491
Other deferred credits and liabilities	150	149
Total deferred credits and other liabilities	4,038	4,006
Commitments and Contingencies (Notes 2, 8, 9 and 10)
Stockholders' Equity:
Common stock	511	511
Other paid-in capital, principally premium on common stock	1,555	1,555
Preferred stock not subject to mandatory redemption	80	80
Retained earnings	1,812	1,891
Total stockholders' equity	3,958	4,037
TOTAL LIABILITIES AND EQUITY	12,546	12,757
Ameren Illinois Company [Member]
Current Assets:
Cash and cash equivalents	187	21
Accounts receivable - trade (less allowance for doubtful accounts)	228	201
Accounts receivable - affiliates	12	15
Unbilled revenue	92	146
Miscellaneous accounts and notes receivable	6	6
Materials and supplies	96	199
Current regulatory assets	316	306
Counterparty collateral asset	70	50
Current accumulated deferred income taxes, net	43	58
Other current assets	11	15
Total current assets	1,061	1,017
Property and Plant, Net	4,804	4,770
Investments and Other Assets:
Tax receivable - Genco	51	56
Goodwill	411	411
Regulatory assets	814	748
Other assets	115	211
Total investments and other assets	1,391	1,426
TOTAL ASSETS	7,256	7,213
Current Liabilities:
Current maturities of long-term debt	1	1
Accounts and wages payable	115	133
Accounts payable - affiliates	98	103
Taxes accrued	15	15
Interest accrued	50	22
Customer deposits	76	76
Mark-to-market derivative liabilities	122	99
Mark-to-market derivative liabilities - affiliates	183	200
Environmental remediation	25	63
Current regulatory liabilities	78	76
Other current liabilities	57	70
Total current liabilities	820	858
Long-term Debt, Net	1,657	1,657
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	934	895
Accumulated deferred investment tax credits	6	7
Regulatory liabilities	608	666
Pension and other postretirement benefits	499	495
Other deferred credits and liabilities	291	183
Total deferred credits and other liabilities	2,338	2,246
Commitments and Contingencies (Notes 2, 8, 9 and 10)
Stockholders' Equity:
Common stock
Other paid-in capital, principally premium on common stock	1,965	1,965
Preferred stock not subject to mandatory redemption	62	62
Retained earnings	398	408
Accumulated other comprehensive income (loss)	16	17
Total stockholders' equity	2,441	2,452
TOTAL LIABILITIES AND EQUITY	7,256	7,213
Ameren Energy Generating Company [Member]
Current Assets:
Cash and cash equivalents		8
Advances to money pool	95	74
Accounts receivable - affiliates	58	89
Miscellaneous accounts and notes receivable	14	13
Materials and supplies	121	122
Mark-to-market derivative assets	13	12
Other current assets	10	7
Total current assets	311	325
Property and Plant, Net	2,242	2,231
Investments and Other Assets:
Other assets	18	16
TOTAL ASSETS	2,571	2,572
Current Liabilities:
Accounts and wages payable	60	71
Accounts payable - affiliates	14	13
Current portion of tax payable - Ameren Illinois	10	8
Taxes accrued	21	20
Interest accrued	27	13
Current accumulated deferred income taxes, net	8
Other current liabilities	18	17
Total current liabilities	158	142
Long-term Debt, Net	824	824
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	291	304
Accumulated deferred investment tax credits	2	2
Tax payable - Ameren Illinois	51	56
Asset retirement obligations	67	66
Pension and other postretirement benefits	140	141
Other deferred credits and liabilities	15	12
Total deferred credits and other liabilities	566	581
Commitments and Contingencies (Notes 2, 8, 9 and 10)
Stockholders' Equity:
Common stock
Other paid-in capital, principally premium on common stock	653	653
Retained earnings	436	437
Accumulated other comprehensive income (loss)	(71)	(72)
Total stockholders' equity	1,018	1,018
Noncontrolling Interests	5	7
Total equity	1,023	1,025
TOTAL LIABILITIES AND EQUITY	$ 2,571	$ 2,572