Consolidated Statement Of Income (Loss) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Operating Revenues:
Electric	$ 6,530		$ 6,521		$ 5,940
Gas	1,001		1,117		1,195
Total operating revenues	7,531		7,638		7,135
Operating Expenses:
Fuel	1,567		1,323		1,141
Purchased power	966		1,106		909
Gas purchased for resale	570		669		749
Other operations and maintenance	1,820		1,821		1,768
Goodwill and other impairment charges	125	[1]	589	[1]	7	[1]
Depreciation and amortization	785		765		725
Taxes other than income taxes	457		449		420
Total operating expenses	6,290		6,722		5,719
Operating income (loss)	1,241		916		1,416
Other Income and Expenses:
Miscellaneous income	69	[2]	90	[2]	71	[2]
Miscellaneous expense	23	[2]	33	[2]	23	[2]
Total other income (expense)	46		57		48
Interest Charges	451		497		508
Income (Loss) Before Income Taxes	836		476		956
Income tax (benefit)	310	[3]	325	[3]	332	[3]
Net Income (Loss)	526		151		624
Less: Net Income Attributable to Noncontrolling Interests	7		12		12
Net Income Attributable to Ameren Corporation	519	[4]	139	[4]	612	[4]
Earnings per Common Share - Basic and Diluted	$ 2.15		$ 0.58		$ 2.78
Dividends per Common Share	$ 1.555		$ 1.540		$ 1.540
Average Common Shares Outstanding	241.5		238.8		220.4
Union Electric Company [Member]
Operating Revenues:
Electric	3,222		3,030		2,700
Gas	156		166		170
Other	5		1		4
Total operating revenues	3,383		3,197		2,874
Operating Expenses:
Fuel	866		635		593
Purchased power	104		162		124
Gas purchased for resale	77		91		97
Other operations and maintenance	934		931		880
Loss from regulatory disallowance	89
Depreciation and amortization	408		382		357
Taxes other than income taxes	296		285		257
Total operating expenses	2,774		2,486		2,308
Operating income (loss)	609		711		566
Other Income and Expenses:
Miscellaneous income	61		83		63
Miscellaneous expense	10		13		7
Total other income (expense)	51		70		56
Interest Charges	209		213		229
Income (Loss) Before Income Taxes	451		568		393
Income tax (benefit)	161		199		128
Net Income (Loss)	290		369		265
Preferred Stock Dividends	3		5		6
Net Income Available to Common Stockholder	287		364		259
Ameren Illinois Company [Member]
Operating Revenues:
Electric	1,940		2,061		1,965	[5]
Gas	846		953		1,015	[5]
Other	1				4	[5]
Total operating revenues	2,787		3,014		2,984	[5]
Operating Expenses:
Purchased power	853		965		1,048	[5]
Gas purchased for resale	492		578		642	[5]
Other operations and maintenance	640		635		590	[5]
Depreciation and amortization	215		210		216	[5]
Taxes other than income taxes	129		128		125	[5]
Total operating expenses	2,329		2,516		2,621	[5]
Operating income (loss)	458		498		363	[5]
Other Income and Expenses:
Miscellaneous income	7		7		12	[5]
Miscellaneous expense	6		13		10	[5]
Total other income (expense)	1		(6)		2	[5]
Interest Charges	136		143		153	[5]
Income (Loss) Before Income Taxes	323		349		212	[5]
Income tax (benefit)	127		137		79	[5]
Income from Continuing Operations	196		212		133	[5]
Income from Discontinued Operations, net of tax			40		114	[5],[6]
Net Income (Loss)	196		252		247	[5],[6]
Preferred Stock Dividends	3		4		6	[5]
Net Income Available to Common Stockholder	193		248		241	[5]
Ameren Energy Generating Company [Member]
Operating Revenues:
Total operating revenues	1,066		1,126		1,148	[7]
Operating Expenses:
Fuel	541		522		415	[7]
Purchased power	55		61		72	[7]
Other operations and maintenance	179		191		226	[7]
Goodwill and other impairment charges	35		170		6	[7]
Depreciation and amortization	96		98		81	[7]
Taxes other than income taxes	21		22		24	[7]
Total operating expenses	927		1,064		824	[7]
Operating income (loss)	139		62		324	[7]
Other Income and Expenses:
Miscellaneous income	1		1		1	[7]
Miscellaneous expense			1		1	[7]
Total other income (expense)	1
Interest Charges	63		78		61	[7]
Income (Loss) Before Income Taxes	77		(16)		263	[7]
Income tax (benefit)	32		20		101	[7]
Net Income (Loss)	45		(36)		162	[7]
Less: Net Income Attributable to Noncontrolling Interests	1		3		2	[7]
Net Income Attributable to Ameren Corporation	$ 44		$ (39)		$ 160	[7]

[1] Includes amounts for registrant and nonregistrant subsidiaries.

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

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

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

[5] Prior period has been adjusted to reflect the AIC Merger as discussed in Note 1 - Summary of Significant Accounting Policies.

[6] Prior period has been adjusted to reflect the Ameren Illinois Merger as discussed in Note 1 - Summary of Significant Accounting Policies.

[7] Prior period has been adjusted to include EEI as discussed in Note 1 - Summary of Significant Accounting Policies.