Segment Information (Details) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment										2
External revenues		$ 1,308	$ 1,833	$ 1,401	$ 1,556	$ 1,370	$ 1,670	$ 1,419	$ 1,594	$ 6,098	$ 6,053	$ 5,838
Depreciation and amortization										796	745	706
Interest Expense										355	341	398
Income taxes (benefit)										363	377	311
Net income (loss) attributable to Ameren common shareholders		29	343	150	108	48	293	149	96	630	586	289
Capital expenditures										1,917	1,785	1,379
TOTAL ASSETS		23,640				22,289				23,640	22,289
Assets of discontinued operations (Note 16)		14				15				14	15	165
Net income (loss) attributable to Ameren common shareholders - continuing operations		30	$ 343	$ 98	$ 108	46	$ 294	$ 150	$ 97	$ 579	587	512
Union Electric Company
Segment Reporting Information [Line Items]
Number of reportable segments										1
External revenues										$ 3,566	3,526	3,516
Revenue from Related Parties										43	27	25
Depreciation and amortization										492	473	454
Interest and dividend income										28	28	27
Interest Expense										219	211	210
Income taxes (benefit)										209	229	242
Net income (loss) attributable to Ameren common shareholders										352	390	395
Capital expenditures										622	747	648
TOTAL ASSETS	[1]	13,851				13,474				$ 13,851	13,474	12,867
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of reportable segments										1
External revenues										$ 2,462	2,496	2,307
Revenue from Related Parties										4	2	4
Depreciation and amortization										295	263	243
Interest and dividend income										12	7	2
Interest Expense										131	112	143
Income taxes (benefit)										127	143	110
Net income (loss) attributable to Ameren common shareholders										214	201	160
Capital expenditures										918	835	701
TOTAL ASSETS	[1]	8,903				8,204				8,903	8,204	7,397
Other Segment
Segment Reporting Information [Line Items]
External revenues										70	31	15
Revenue from Related Parties										2	2	2
Depreciation and amortization										9	9	9
Interest and dividend income										1	2	1
Interest Expense										5	18	45
Income taxes (benefit)										27	5	(41)
Net income (loss) attributable to Ameren common shareholders										13	(4)	(43)
Capital expenditures	[2]									377	203	30
TOTAL ASSETS	[1]	1,139				799				1,139	799	711
Intersegment Elimination
Segment Reporting Information [Line Items]
External revenues										0	0	0
Revenue from Related Parties										(49)	(31)	(31)
Depreciation and amortization										0	0	0
Interest and dividend income										0	0	0
Interest Expense										0	0	0
Income taxes (benefit)										0	0	0
Net income (loss) attributable to Ameren common shareholders										0	0	0
Capital expenditures										0	0	0
TOTAL ASSETS		(267)				(203)				(267)	(203)	(233)
Segment, Continuing Operations
Segment Reporting Information [Line Items]
External revenues										6,098	6,053	5,838
Revenue from Related Parties										0	0	0
Depreciation and amortization										796	745	706
Interest and dividend income										41	37	30
Interest Expense										355	341	398
Income taxes (benefit)										363	377	311
Capital expenditures										1,917	1,785	1,379
TOTAL ASSETS	[1],[3]	$ 23,626				$ 22,274				$ 23,626	$ 22,274	$ 20,742

[1] (c)Reflects the adoption of the new authoritative accounting guidance for the presentation of debt issuance costs and balance sheet classification of deferred income taxes. See Note 1 – Summary of Significant Accounting Policies for additional information.

[2] (a)Includes the elimination of intercompany transfers.

[3] (b)Excludes total assets from discontinued operations of $14 million, $15 million, and $165 million as of December 31, 2015, 2014, and 2013, respectively.