Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 segment		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information [Line Items]
Assets of discontinued operations (Note 16)	$ 165					$ 1,611					$ 165		$ 1,611		$ 3,721
Number of reportable segments											2
External revenues	1,322		1,638	1,403	1,475	1,258		1,709	1,402	1,412	5,838		5,781		6,148
Depreciation and amortization											706		673		643
Interest Expense											398		392		387
Income taxes (benefit)											311	[1]	307	[1]	254	[1]
Net income (loss) attributable to Ameren Corporation	37		302	95	(145)	(1,156)		374	211	(403)	289		(974)		519
Income (Loss) from Continuing Operations Attributable to Parent	48		305	105	54	12		302	164	38	512		516		431
Capital expenditures											1,379		1,063		881
Total assets	21,042					22,230					21,042		22,230
Union Electric Company
Segment Reporting Information [Line Items]
External revenues											3,516		3,252		3,360
Revenue from Related Parties											25		20		23
Depreciation and amortization											454		440		408
Interest and dividend income											27		32		30
Interest Expense											210		223		209
Income taxes (benefit)											242		252		161
Net income (loss) attributable to Ameren Corporation											395		416		287
Capital expenditures											648		595		550
Total assets	12,904					13,043					12,904		13,043		12,757
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues											2,307		2,524		2,784
Revenue from Related Parties											4		1		3
Depreciation and amortization											243		221		215
Interest and dividend income											2		0		1
Interest Expense											143		129		136
Income taxes (benefit)											110		94		127
Net income (loss) attributable to Ameren Corporation											160		141		193
Capital expenditures											701		442		351
Total assets	7,454					7,282					7,454		7,282		7,213
Other Segment
Segment Reporting Information [Line Items]
External revenues											15		5		4
Revenue from Related Parties											2		3		3
Depreciation and amortization											9		12		20
Interest and dividend income											1
Interest Expense											45		40		42
Income taxes (benefit)											(41)		(39)		(34)
Net income (loss) attributable to Ameren Corporation											(43)		(41)		(49)
Capital expenditures											30	[2]	26	[2]	(20)	[2]
Total assets	752					1,228					752		1,228		1,211
Intersegment Elimination
Segment Reporting Information [Line Items]
Revenue from Related Parties											(31)		(24)		(29)
Interest and dividend income
Interest Expense
Total assets	(233)					(934)					(233)		(934)		(1,179)
Segment, Continuing Operations
Segment Reporting Information [Line Items]
External revenues											5,838		5,781		6,148
Depreciation and amortization											706		673		643
Interest and dividend income											30		32		31
Interest Expense											398		392		387
Income taxes (benefit)											311		307		254
Income (Loss) from Continuing Operations Attributable to Parent											512		516		431
Capital expenditures											1,379		1,063		881
Total assets	20,877	[3]				20,619	[3]				20,877	[3]	20,619	[3]	20,002	[3]
Union Electric Company
Segment Reporting Information [Line Items]
External revenues	763		1,093	889	796	673		1,064	844	691	3,541		3,272		3,383
Revenue from Related Parties											25		20		23
Depreciation and amortization											454		440		408
Interest Expense											210		223		209
Income taxes (benefit)											242		252		161
Net income (loss) attributable to Ameren Corporation	33		239	85	41	16		237	144	22	398		419		290
Capital expenditures											648		595		550
Total assets	12,904					13,043					12,904		13,043
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	564		547	516	684	589		648	564	724	2,311		2,525		2,787
Revenue from Related Parties											4		1		3
Depreciation and amortization											243		221		215
Interest Expense											143		129		136
Income taxes (benefit)											110		94		127
Net income (loss) attributable to Ameren Corporation	22		77	32	32	12		71	33	28	163		144		196
Capital expenditures											701		442		351
Total assets	$ 7,454					$ 7,282					$ 7,454		$ 7,282

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

[2] Includes the elimination of intercompany transfers.

[3] Excludes total assets from discontinued operations of $165 million, $1,611 million, and $3,721 million as of December 31, 2013, 2012, and 2011, respectively.