Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment									4
External revenues	$ 1,402	$ 1,723	$ 1,538	$ 1,514	$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 6,177	$ 6,076	$ 6,098
Intersegment revenues									0	0	0
Depreciation and amortization									896	845	796
Interest income									34	40	41
Interest charges									391	382	355
Income taxes (benefit)									576	382	363
Net income (loss) attributable to Ameren common shareholders from continuing operations									523	653	579
Capital expenditures									$ 2,132	2,076	1,917
Union Electric Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment									1
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment									3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	674	575	576	703	595	676	542	677	$ 2,528	2,490	2,466
Intersegment revenues									0	0	0
Depreciation and amortization									341	319	295
Interest income									7	12	12
Interest charges									144	140	131
Income taxes (benefit)									166	158	127
Net Income (Loss) Available to Common Shareholder	$ 77	$ 55	$ 57	$ 79	$ 29	$ 119	$ 45	$ 59	268	252	214
Capital expenditures									1,076	924	918
Intersegment Elimination
Segment Reporting Information [Line Items]
External revenues									0	0	0
Intersegment revenues									(98)	(105)	(87)
Depreciation and amortization									0	0	0
Interest income									(11)	(11)	(6)
Interest charges									(11)	(11)	(6)
Income taxes (benefit)									0	0	0
Net Income (Loss) Available to Common Shareholder									0	0	0
Capital expenditures									(7)	(6)	0
Intersegment Elimination | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues									0	0	0
Intersegment revenues									(42)	(45)	(38)
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									3,490	3,469	3,566
Intersegment revenues									49	54	43
Depreciation and amortization									533	514	492
Interest income									27	28	28
Interest charges									207	211	219
Income taxes (benefit)									254	216	209
Net Income (Loss) Available to Common Shareholder									323	357	352
Capital expenditures									773	738	622
Operating Segments [Member] | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues									1,565	1,545	1,529
Intersegment revenues									4	4	3
Depreciation and amortization									239	226	212
Interest income									7	11	12
Interest charges									73	72	71
Income taxes (benefit)									83	78	71
Net Income (Loss) Available to Common Shareholder									131	126	123
Capital expenditures									476	470	491
Operating Segments [Member] | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues									742	753	782
Intersegment revenues									1	1	1
Depreciation and amortization									59	55	52
Interest income									0	0	0
Interest charges									36	34	35
Income taxes (benefit)									36	39	24
Net Income (Loss) Available to Common Shareholder									60	59	37
Capital expenditures									245	181	133
Operating Segments [Member] | Ameren Transmission [Member]
Segment Reporting Information [Line Items]
External revenues									382	309	219
Intersegment revenues									44	46	40
Depreciation and amortization									60	43	33
Interest income									0	1	0
Interest charges									67	58	35
Income taxes (benefit)									90	74	51
Net Income (Loss) Available to Common Shareholder									140	117	83
Capital expenditures									644	689	669
Operating Segments [Member] | Other Segment
Segment Reporting Information [Line Items]
External revenues									(2)	0	2
Intersegment revenues									0	0	0
Depreciation and amortization									5	7	7
Interest income									11	11	7
Interest charges									19	18	1
Income taxes (benefit)									113	(25)	8
Net Income (Loss) Available to Common Shareholder									(131)	(6)	(16)
Capital expenditures									1	4	2
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues									1,569	1,549	1,532
Intersegment revenues									0	0	0
Depreciation and amortization									239	226	212
Interest income									7	11	12
Interest charges									73	72	71
Income taxes (benefit)									83	78	71
Net Income (Loss) Available to Common Shareholder									131	126	123
Capital expenditures									476	470	491
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues									743	754	783
Intersegment revenues									0	0	0
Depreciation and amortization									59	55	52
Interest income									0	0	0
Interest charges									36	34	35
Income taxes (benefit)									36	39	24
Net Income (Loss) Available to Common Shareholder									60	59	37
Capital expenditures									245	181	133
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Segment Reporting Information [Line Items]
External revenues									216	187	151
Intersegment revenues									42	45	38
Depreciation and amortization									43	38	31
Interest income									0	1	0
Interest charges									35	34	25
Income taxes (benefit)									47	41	32
Net Income (Loss) Available to Common Shareholder									77	67	54
Capital expenditures									$ 355	$ 273	$ 294