Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 segment		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of reportable segments											2
External revenues	$ 1,370		$ 1,670	$ 1,419	$ 1,594	$ 1,322		$ 1,638	$ 1,403	$ 1,475	$ 6,053		$ 5,838		$ 5,781
Depreciation and amortization											745		706		673
Interest Expense											341		398		392
Income taxes (benefit)											377		311		307
Net income (loss) attributable to Ameren Corporation	48		293	149	96	37		302	95	(145)	586		289		(974)
Capital expenditures											1,785		1,379		1,063
Total assets	22,676					21,042					22,676		21,042
Assets of discontinued operations (Note 16)	15					165					15		165		1,611
Income (Loss) from Continuing Operations Attributable to Parent	46		294	150	97	48		305	105	54	587		512		516
Union Electric Company
Segment Reporting Information [Line Items]
External revenues											3,526		3,516		3,252
Revenue from Related Parties											27		25		20
Depreciation and amortization											473		454		440
Interest and dividend income											28		27		32
Interest Expense											211		210		223
Income taxes (benefit)											229		242		252
Net income (loss) attributable to Ameren Corporation											390		395		416
Capital expenditures											747		648		595
Total assets	13,541					12,904					13,541		12,904		13,043
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues											2,496		2,307		2,524
Revenue from Related Parties											2		4		1
Depreciation and amortization											263		243		221
Interest and dividend income											7		2		0
Interest Expense											112		143		129
Income taxes (benefit)											143		110		94
Net income (loss) attributable to Ameren Corporation											201		160		141
Capital expenditures											835		701		442
Total assets	8,381					7,454					8,381		7,454		7,282
Other Segment
Segment Reporting Information [Line Items]
External revenues											31		15		5
Revenue from Related Parties											2		2		3
Depreciation and amortization											9		9		12
Interest and dividend income											2		1
Interest Expense											18		45		40
Income taxes (benefit)											5		(41)		(39)
Net income (loss) attributable to Ameren Corporation											(4)		(43)		(41)
Capital expenditures											203	[1]	30	[1]	26	[1]
Total assets	942					752					942		752		1,228
Intersegment Elimination
Segment Reporting Information [Line Items]
Revenue from Related Parties											(31)		(31)		(24)
Total assets	(203)					(233)					(203)		(233)		(934)
Segment, Continuing Operations
Segment Reporting Information [Line Items]
External revenues											6,053		5,838		5,781
Depreciation and amortization											745		706		673
Interest and dividend income											37		30		32
Interest Expense											341		398		392
Income taxes (benefit)											377		311		307
Capital expenditures											1,785		1,379		1,063
Total assets	22,661	[2]				20,877	[2]				22,661	[2]	20,877	[2]	20,619	[2]
Union Electric Company
Segment Reporting Information [Line Items]
External revenues	739		1,097	900	817	763		1,093	889	796	3,553		3,541		3,272
Revenue from Related Parties											27		25		20
Depreciation and amortization											473		454		440
Interest Expense											211		210		223
Income taxes (benefit)											229		242		252
Net income (loss) attributable to Ameren Corporation	(5)		223	127	48	33		239	85	41	393		398		419
Capital expenditures											747		648		595
Total assets	13,541					12,904					13,541		12,904
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	633		572	519	774	564		547	516	684	2,498		2,311		2,525
Revenue from Related Parties											2		4		1
Depreciation and amortization											263		243		221
Interest Expense											112		143		129
Income taxes (benefit)											143		110		94
Net income (loss) attributable to Ameren Corporation	46		75	29	54	22		77	32	32	204		163		144
Capital expenditures											835		701		442
Total assets	$ 8,381					$ 7,454					$ 8,381		$ 7,454

[1] Includes the elimination of intercompany transfers.

[2] Excludes total assets from discontinued operations of $15 million, $165 million, and $1,611 million as of December 31, 2014, 2013, and 2012, respectively.