Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012 segment		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Number of reportable segments																	3
External revenues	$ 1,509	[1]	$ 2,001	[1]	$ 1,660	[1]	$ 1,658	[1]	$ 1,578	[1]	$ 2,268	[1]	$ 1,781	[1]	$ 1,904	[1]	$ 6,828		$ 7,531		$ 7,638
Depreciation and amortization																	775		785		765
Interest and dividend income																	33		32		33
Interest and dividend income																	5	[2],[3]	4	[2]	5	[2]
Interest Expense																	448		451		497
Income taxes (benefit)																	(680)	[2]	310	[2]	325	[2]
Net income (loss) attributable to Ameren Corporation	(1,156)	[1]	374	[1]	211	[1]	(403)	[1]	25	[1]	285	[1]	138	[1]	71	[1]	(974)	[4]	519	[4]	139	[4]
Capital expenditures																	1,240		1,030		1,042
Total assets	21,835								23,645								21,835		23,645		23,511
Impairment and other charges																	2,578	[5]	125	[5]	589	[5]
Union Electric Company [Member]
Segment Reporting Information [Line Items]
External revenues																	3,251		3,358		3,176
Intersegment revenues																	21		25		21
Depreciation and amortization																	440		408		382
Interest and dividend income																	32		30		31
Interest Expense																	223		209		213
Income taxes (benefit)																	252		161		199
Net income (loss) attributable to Ameren Corporation																	416	[4]	287	[4]	364	[4]
Capital expenditures																	595		550		624
Total assets	13,043								12,757								13,043		12,757		12,504
Ameren Illinois Company [Member]
Segment Reporting Information [Line Items]
External revenues																	2,509		2,774		3,002
Intersegment revenues																	16		13		12
Depreciation and amortization																	221		215		210
Interest and dividend income																			1		1
Interest and dividend income
Interest Expense																	129		136		143
Income taxes (benefit)																	94		127		137
Net income (loss) attributable to Ameren Corporation																	141	[4]	193	[4]	208	[4]
Capital expenditures																	442		351		281
Total assets	7,282								7,213								7,282		7,213		7,406
Merchant Generation [Member]
Segment Reporting Information [Line Items]
External revenues																	1,063		1,394		1,459
Intersegment revenues																	310		235		234
Depreciation and amortization																	102		143		146
Interest and dividend income																					1
Interest Expense																	95		105		133
Income taxes (benefit)																	(1,019)		32		6
Net income (loss) attributable to Ameren Corporation																	(1,516)	[4],[6]	45	[4]	(409)	[4],[6]
Capital expenditures																	178		153		101
Total assets	1,300								3,833								1,300		3,833		3,934
Impairment and other charges																	2,578				589
Other Segment [Member]
Segment Reporting Information [Line Items]
External revenues																	5		5		1
Intersegment revenues																	4		4		13
Depreciation and amortization																	12		19		27
Interest and dividend income																	40		44		25
Interest Expense																	38		44		35
Income taxes (benefit)																	(7)		(10)		(17)
Net income (loss) attributable to Ameren Corporation																	(15)	[4]	(6)	[4]	(24)	[4]
Capital expenditures																	25		(24)	[7]	36
Total assets	1,228								1,211								1,228		1,211		1,354
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Intersegment revenues																	(351)		(277)		(280)
Interest and dividend income																	(39)		(43)		(25)
Interest Expense																	(37)		(43)		(27)
Total assets	(1,018)								(1,369)								(1,018)		(1,369)		(1,687)
Union Electric Company [Member]
Segment Reporting Information [Line Items]
External revenues	673		1,064		844		691		674		1,115		822		772		3,272		3,383		3,197
Depreciation and amortization																	440		408		382
Interest and dividend income																	4	[3]	2		3
Interest Expense																	223		209		213
Income taxes (benefit)																	252		161		199
Net income (loss) attributable to Ameren Corporation	16		237		144		22		(14)		191		91		22		419		290		369
Capital expenditures																	595		550		624
Total assets	13,043								12,757								13,043		12,757
Ameren Illinois Company [Member]
Segment Reporting Information [Line Items]
External revenues	589		648		564		724		611		745		623		808		2,525		2,787		3,014
Depreciation and amortization																	221		215		210
Interest and dividend income																			1		1
Interest Expense																	129		136		143
Income taxes (benefit)																	94		127		137
Net income (loss) attributable to Ameren Corporation	12		71		33		28		26		98		38		34		144		196		252
Capital expenditures																	442		351		281
Total assets	$ 7,282								$ 7,213								$ 7,282		$ 7,213

[1] The sum of quarterly amounts, including per share amounts, may not equal amounts reported for year-to-date periods. This is due to the effects of rounding and changes in the number of weighted-average shares outstanding each period.

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

[3] Includes interest income relating to a 2012 refund of charges included in an expired power purchase agreement with Entergy. See Note 2 - Rate and Regulatory Matters for additional information.

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

[5] Includes amounts for registrant and nonregistrant subsidiaries.

[6] Includes noncash impairment and other charges, which were $2,578 million and $589 million before tax, recognized during the years ended December 31, 2012, and 2010, respectively. See Note 17 - Impairment and Other Charges for additional information

[7] Includes the elimination of intercompany transfers.