Segment Information (Details) $ in Millions		3 Months Ended								9 Months Ended	12 Months Ended
		Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2016 segment	Dec. 31, 2016 USD ($) segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment										2	4
External revenues		$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 1,308	$ 1,833	$ 1,401	$ 1,556		$ 6,076	$ 6,098	$ 6,053
Depreciation and amortization											845	796	745
Interest charges											382	355	341
Income taxes (benefit)											382	363	377
Net income (loss) attributable to Ameren common shareholders from continuing operations		32	369	147	105	30	343	98	108		653	579	587
Capital expenditures											$ 2,076	1,917	1,785
Union Electric Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment											1
Segment, Continuing Operations
Segment Reporting Information [Line Items]
External revenues											$ 6,076	6,098	6,053
Intersegment revenues											0	0	0
Depreciation and amortization											845	796	745
Interest income											40	41	37
Interest charges											382	355	341
Income taxes (benefit)											382	363	377
Net income (loss) attributable to Ameren common shareholders from continuing operations											653	579	587
Capital expenditures											$ 2,076	1,917	1,785
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment											3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		595	676	542	677	553	655	513	745		$ 2,490	2,466	2,498
Intersegment revenues											0	0	0
Depreciation and amortization											319	295	263
Interest income											12	12	7
Interest charges											140	131	112
Income taxes (benefit)											158	127	143
Net Income (Loss) Available to Common Shareholder		$ 29	$ 119	$ 45	$ 59	$ 32	$ 98	$ 31	$ 53		252	214	201
Capital expenditures											924	918	835
Intersegment Elimination
Segment Reporting Information [Line Items]
External revenues											0	0	0
Intersegment revenues											(105)	(87)	(66)
Depreciation and amortization											0	0	0
Interest income											(11)	(6)	(3)
Interest charges											(11)	(6)	(3)
Income taxes (benefit)											0	0	0
Net Income (Loss) Available to Common Shareholder											0	0	0
Capital expenditures											(6)	0	0
Intersegment Elimination | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues											0	0	0
Intersegment revenues											(45)	(38)	(35)
Depreciation and amortization											0	0	0
Interest income											0	0	0
Interest charges											0	0	0
Income taxes (benefit)											0	0	0
Net Income (Loss) Available to Common Shareholder											0	0	0
Capital expenditures											0	0	0
Operating Segments [Member] | Union Electric Company
Segment Reporting Information [Line Items]
External revenues											3,469	3,566	3,526
Intersegment revenues											54	43	27
Depreciation and amortization											514	492	473
Interest income											28	28	28
Interest charges											211	219	211
Income taxes (benefit)											216	209	229
Net Income (Loss) Available to Common Shareholder											357	352	390
Capital expenditures											738	622	747
Operating Segments [Member] | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues											1,545	1,529	1,401
Intersegment revenues											4	3	2
Depreciation and amortization											226	212	197
Interest income											11	12	7
Interest charges											72	71	63
Income taxes (benefit)											78	71	75
Net Income (Loss) Available to Common Shareholder											126	123	113
Capital expenditures											470	491	403
Operating Segments [Member] | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues											753	782	976
Intersegment revenues											1	1	0
Depreciation and amortization											55	52	41
Interest income											0	0	0
Interest charges											34	35	28
Income taxes (benefit)											39	24	39
Net Income (Loss) Available to Common Shareholder											59	37	50
Capital expenditures											181	133	137
Operating Segments [Member] | Ameren Transmission [Member]
Segment Reporting Information [Line Items]
External revenues											309	219	150
Intersegment revenues	[1]										46	40	37
Depreciation and amortization											43	33	26
Interest income											1	0	0
Interest charges											58	35	26
Income taxes (benefit)											74	51	38
Net Income (Loss) Available to Common Shareholder											117	83	51
Capital expenditures											689	669	491
Operating Segments [Member] | Other Segment
Segment Reporting Information [Line Items]
External revenues											0	2	0
Intersegment revenues											0	0	0
Depreciation and amortization											7	7	8
Interest income											11	7	5
Interest charges											18	1	16
Income taxes (benefit)											(25)	8	(4)
Net Income (Loss) Available to Common Shareholder											(6)	(16)	(17)
Capital expenditures	[2]										4	2	7
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues											1,549	1,532	1,403
Intersegment revenues											0	0	0
Depreciation and amortization											226	212	197
Interest income											11	12	7
Interest charges											72	71	63
Income taxes (benefit)											78	71	75
Net Income (Loss) Available to Common Shareholder											126	123	113
Capital expenditures											470	491	403
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues											754	783	976
Intersegment revenues											0	0	0
Depreciation and amortization											55	52	41
Interest income											0	0	0
Interest charges											34	35	28
Income taxes (benefit)											39	24	39
Net Income (Loss) Available to Common Shareholder											59	37	50
Capital expenditures											181	133	137
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Segment Reporting Information [Line Items]
External revenues											187	151	119
Intersegment revenues	[3]										45	38	35
Depreciation and amortization											38	31	25
Interest income											1	0	0
Interest charges											34	25	21
Income taxes (benefit)											41	32	29
Net Income (Loss) Available to Common Shareholder											67	54	38
Capital expenditures											$ 273	$ 294	$ 295

[1] (a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.

[2] b)Includes the elimination of intercompany transfers.

[3] (a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.