Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,724	$ 1,723	$ 4,872	$ 4,775
Operating Expenses:
Fuel	216	199	590	594
Utilities Operating Expense, Purchased Power	148	163	453	493
Utilities Operating Expense, Gas and Petroleum Purchased	30	25	252	196
Other operations and maintenance	429	413	1,299	1,262
Depreciation and amortization	241	225	713	668
Taxes other than income taxes	127	129	374	364
Total operating expenses	1,191	1,154	3,681	3,577
Operating Income	533	569	1,191	1,198
Other Income (Expense), Net	32	23	84	61
Interest Charges	101	97	302	295
Income Before Income Taxes	464	495	973	964
Income Taxes	105	205	221	376
Net income	359	290	752	588
Less: Net Income Attributable to Noncontrolling Interests	2	2	5	5
Net Income (Loss) Available to Common Stockholders, Basic	357	288	747	583
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	2	0	1	2
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	361	290	753	590
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2	5	5
Comprehensive Income	$ 359	$ 288	$ 748	$ 585
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 1.46	$ 1.19	$ 3.06	$ 2.40
Earnings Per Share, Diluted	1.45	1.18	3.04	2.39
Dividends per Common Share (in dollars per share)	$ 0.4575	$ 0.44	$ 1.3725	$ 1.320
Average Common Shares Outstanding - Basic (in shares)	244.1	242.6	243.6	242.6
Weighted Average Number of Shares Outstanding, Diluted	246.3	244.7	245.5	244.0
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,590	$ 1,594	$ 4,209	$ 4,183
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	134	129	663	592
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,129	1,116	2,876	2,841
Operating Expenses:
Fuel	216	199	590	594
Utilities Operating Expense, Purchased Power	49	43	131	203
Utilities Operating Expense, Gas and Petroleum Purchased	5	4	37	29
Other operations and maintenance	234	229	707	672
Depreciation and amortization	137	134	411	399
Taxes other than income taxes	94	95	258	255
Total operating expenses	735	704	2,134	2,152
Operating Income	394	412	742	689
Other Income (Expense), Net	16	16	45	48
Interest Charges	50	50	152	157
Income Before Income Taxes	360	378	635	580
Income Taxes	65	143	132	218
Net income	295	235	503	362
Preferred Stock Dividends	1	1	3	3
Net Income (Loss) Attributable to Parent	294	234	500	359
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,111	1,099	2,782	2,758
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	18	17	94	83
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	564	574	1,902	1,853
Operating Expenses:
Utilities Operating Expense, Purchased Power	105	124	334	312
Utilities Operating Expense, Gas and Petroleum Purchased	25	21	215	167
Other operations and maintenance	195	186	590	598
Depreciation and amortization	94	86	278	254
Taxes other than income taxes	32	33	108	101
Total operating expenses	451	450	1,525	1,432
Operating Income	113	124	377	421
Other Income (Expense), Net	11	5	30	8
Interest Charges	38	36	112	109
Income Before Income Taxes	86	93	295	320
Income Taxes	23	38	73	127
Net income	63	55	222	193
Preferred Stock Dividends	0	0	2	2
Net Income (Loss) Attributable to Parent	63	55	220	191
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	448	462	1,333	1,343
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 116	$ 112	$ 569	$ 510