Consolidated Statement Of Income (Loss) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Operating Revenues:
Electric	$ 5,904		$ 6,530		$ 6,521
Gas	924		1,001		1,117
Total operating revenues	6,828		7,531		7,638
Operating Expenses:
Fuel	1,369		1,567		1,323
Purchased power	654		966		1,106
Gas purchased for resale	472		570		669
Other operations and maintenance	1,752		1,820		1,821
Impairment and other charges	2,578	[1]	125	[1]	589	[1]
Depreciation and amortization	775		785		765
Taxes other than income taxes	468		457		449
Total operating expenses	8,068		6,290		6,722
Operating Income (Loss)	(1,240)		1,241		916
Other Income and Expenses:
Miscellaneous expense	71	[2]	69	[2]	90	[2]
Miscellaneous expense	37	[2]	23	[2]	33	[2]
Total other income (expense)	34		46		57
Interest Charges	448		451		497
Income (Loss) Before Income Taxes	(1,654)		836		476
Income taxes (benefit)	(680)	[2]	310	[2]	325	[2]
Net Income (Loss)	(974)		526		151
Comprehensive Income (Loss)	(932)		486		135
Less: Net Income Attributable to Noncontrolling Interests			7		12
Net income (loss)	(974)	[3]	519	[3]	139	[3]
Earnings per Common Share - Basic and Diluted	$ (4.01)		$ 2.15		$ 0.58
Dividends per Common Share	$ 1.600		$ 1.555		$ 1.540
Average Common Shares Outstanding	242.6		241.5		238.8
Union Electric Company [Member]
Operating Revenues:
Electric	3,132		3,222		3,030
Gas	139		156		166
Other	1		5		1
Total operating revenues	3,272		3,383		3,197
Operating Expenses:
Fuel	714		866		635
Purchased power	78		104		162
Gas purchased for resale	64		77		91
Other operations and maintenance	827		934		931
Loss from regulatory disallowance			89
Depreciation and amortization	440		408		382
Taxes other than income taxes	304		296		285
Total operating expenses	2,427		2,774		2,486
Operating Income (Loss)	845		609		711
Other Income and Expenses:
Miscellaneous expense	63		61		83
Miscellaneous expense	14		10		13
Total other income (expense)	49		51		70
Interest Charges	223		209		213
Income (Loss) Before Income Taxes	671		451		568
Income taxes (benefit)	252		161		199
Net Income (Loss)	419		290		369
Other Comprehensive Income
Comprehensive Income (Loss)	419		290		369
Net income (loss)	419		290		369
Preferred Stock Dividends	3		3		5
Net Income Available to Common Stockholder	416		287		364
Ameren Illinois Company [Member]
Operating Revenues:
Electric	1,739		1,940		2,061
Gas	786		846		953
Other			1
Total operating revenues	2,525		2,787		3,014
Operating Expenses:
Purchased power	705		853		965
Gas purchased for resale	408		492		578
Other operations and maintenance	684		640		635
Depreciation and amortization	221		215		210
Taxes other than income taxes	130		129		128
Total operating expenses	2,148		2,329		2,516
Operating Income (Loss)	377		458		498
Other Income and Expenses:
Miscellaneous expense	7		7		7
Miscellaneous expense	17		6		13
Total other income (expense)	(10)		1		(6)
Interest Charges	129		136		143
Income (Loss) Before Income Taxes	238		323		349
Income taxes (benefit)	94		127		137
Income from Continuing Operations	144		196		212
Income from discontinued operations, net of tax			0		40
Net Income (Loss)	144		196		252
Pension and other postretirement benefit plan activity, net of tax benefit	(3)		(3)		(4)
Other comprehensive income from discontinued operations					(1)
Comprehensive Income (Loss)	141		193		247
Net income (loss)	144		196		252
Preferred Stock Dividends	3		3		4
Net Income Available to Common Stockholder	$ 141		$ 193		$ 248

[1] Includes amounts for registrant and nonregistrant subsidiaries.

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

[3] Represents net income (loss) available to common stockholders.