Consolidated Balance Sheet (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Current Assets:
Cash and cash equivalents	$ 161		$ 184
Accounts receivable - trade (less allowance for doubtful accounts)	479		354
Unbilled revenue	262		291
Miscellaneous accounts and notes receivable	70		71
Materials and supplies	461		570
Mark-to-market derivative assets	27		23
Current regulatory assets	196		247
Current accumulated deferred income taxes, net	80		160
Other current assets	60		75
Current assets of discontinued operations	1,500		1,600
Total current assets	3,296		3,575
Property and Plant, Net	15,408		15,348
Investments and Other Assets:
Nuclear decommissioning trust fund	437		408
Goodwill	411		411
Intangible assets	16		14
Regulatory assets	1,719		1,785
Other assets	665		668
Total investments and other assets	3,248		3,286
TOTAL ASSETS	21,952		22,209
Current Liabilities:
Current maturities of long-term debt	355		355
Accounts and wages payable	341		533
Taxes accrued	91		50
Interest accrued	111		89
Customer deposits	110		107
Mark-to-market derivative liabilities	73		92
Current regulatory liabilities	135		100
Other current liabilities	163		168
Current liabilities of discontinued operations	1,198		1,166
Total current liabilities	2,577		2,660
Long-term Debt, Net	5,803		5,802
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	3,235		3,166
Accumulated deferred investment tax credits	68		70
Regulatory liabilities	1,667		1,589
Asset retirement obligations	380		375
Pension and other postretirement benefits	1,113		1,138
Other deferred credits and liabilities	593		642
Total deferred credits and other liabilities	7,056		6,980
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2		2
Other paid-in capital	5,614		5,616
Retained earnings	764		1,006
Accumulated other comprehensive gain (loss)	(15)		(8)
Stockholder's equity	6,365		6,616
Noncontrolling Interest	151	[1]	151	[1]
Total equity	6,516		6,767
TOTAL LIABILITIES AND EQUITY	21,952		22,209
Union Electric Company [Member]
Current Assets:
Cash and cash equivalents	1		148
Advances to money pool			24
Accounts receivable - trade (less allowance for doubtful accounts)	202		161
Accounts receivable - affiliates	6		4
Unbilled revenue	151		145
Miscellaneous accounts and notes receivable	49		48
Materials and supplies	375		397
Current regulatory assets	144		163
Other current assets	51		69
Total current assets	979		1,159
Property and Plant, Net	10,152		10,161
Investments and Other Assets:
Nuclear decommissioning trust fund	437		408
Intangible assets	16		14
Regulatory assets	832		852
Other assets	451		449
Total investments and other assets	1,736		1,723
TOTAL ASSETS	12,867		13,043
Current Liabilities:
Current maturities of long-term debt	205		205
Borrowings from money pool	5
Accounts and wages payable	153		345
Accounts payable - affiliates	65		66
Taxes accrued	66		28
Interest accrued	53		60
Current regulatory liabilities	23		18
Other current liabilities	86		77
Total current liabilities	656		799
Long-term Debt, Net	3,801		3,801
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,426		2,443
Accumulated deferred investment tax credits	63		64
Regulatory liabilities	968		917
Asset retirement obligations	351		346
Pension and other postretirement benefits	452		461
Other deferred credits and liabilities	146		158
Total deferred credits and other liabilities	4,406		4,389
Commitments and Contingencies
Stockholders' Equity:
Common Stock	511		511
Other paid-in capital	1,556		1,556
Preferred stock not subject to mandatory redemption	80		80
Retained earnings	1,857		1,907
Stockholder's equity	4,004		4,054
TOTAL LIABILITIES AND EQUITY	12,867		13,043
Ameren Illinois Company [Member]
Current Assets:
Cash and cash equivalents	93		0
Advances to money pool	5
Accounts receivable - trade (less allowance for doubtful accounts)	274		182
Accounts receivable - affiliates	13		10
Unbilled revenue	111		146
Miscellaneous accounts and notes receivable	21		22
Materials and supplies	86		173
Current regulatory assets	52		84
Current accumulated deferred income taxes, net	56		85
Other current assets	36		47
Total current assets	747		749
Property and Plant, Net	5,117		5,052
Investments and Other Assets:
Tax receivable - Genco	38		39
Goodwill	411		411
Regulatory assets	887		934
Other assets	88		97
Total investments and other assets	1,424		1,481
TOTAL ASSETS	7,288		7,282
Current Liabilities:
Current maturities of long-term debt	150		150
Borrowings from money pool			24
Accounts and wages payable	151		146
Accounts payable - affiliates	74		86
Taxes accrued	21		18
Interest accrued	41		22
Customer deposits	86		85
Mark-to-market derivative liabilities	49		77
Environmental remediation	44		37
Current regulatory liabilities	113		82
Other current liabilities	63		70
Total current liabilities	792		797
Long-term Debt, Net	1,577		1,577
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,046		1,025
Accumulated deferred investment tax credits	5		5
Regulatory liabilities	699		672
Pension and other postretirement benefits	396		406
Accrued Environmental Loss Contingencies, Noncurrent	211		216
Other deferred credits and liabilities	146		183
Total deferred credits and other liabilities	2,503		2,507
Commitments and Contingencies
Stockholders' Equity:
Common Stock
Other paid-in capital	1,965		1,965
Preferred stock not subject to mandatory redemption	62		62
Retained earnings	376		360
Accumulated other comprehensive gain (loss)	13		14
Stockholder's equity	2,416		2,401
TOTAL LIABILITIES AND EQUITY	$ 7,288		$ 7,282

[1] Includes the 20% EEI ownership interest not owned by Ameren. The assets and liabilities of EEI were consolidated in Ameren’s balance sheet at a 100% ownership level and were included in “Current assets of discontinued operations” and “Current liabilities of discontinued operations,” however, the 20% ownership interest not owned by Ameren was included in “Noncontrolling interests” on Ameren’s March 31, 2013, and December 31, 2012 balance sheet. See Note 2 - Divestiture Transactions and Discontinued Operations for additional information.